Reland "Enable Video-QualityScaling experiment by default"

This time exclude iOS from the default behaviour.

Bug: webrtc:12401
Change-Id: Ib1f77123b72c3365591b56455332b3d97b307b26
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/205006
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33173}

modules/video_coding/utility/quality_scaler_unittest.cc

@@ -116,7 +116,7 @@ INSTANTIATE_TEST_SUITE_P(
     QualityScalerTest,
     ::testing::Values(
         "WebRTC-Video-QualityScaling/Enabled-1,2,3,4,5,6,7,8,0.9,0.99,1/",
-        ""));
+        "WebRTC-Video-QualityScaling/Disabled/"));
 
 TEST_P(QualityScalerTest, DownscalesAfterContinuousFramedrop) {
   task_queue_.SendTask([this] { TriggerScale(kScaleDown); }, RTC_FROM_HERE);
@@ -171,7 +171,8 @@ TEST_P(QualityScalerTest, DoesNotDownscaleAfterHalfFramedrop) {
 }
 
 TEST_P(QualityScalerTest, DownscalesAfterTwoThirdsIfFieldTrialEnabled) {
-  const bool kDownScaleExpected = !GetParam().empty();
+  const bool kDownScaleExpected =
+      GetParam().find("Enabled") != std::string::npos;
   task_queue_.SendTask(
       [this] {
         for (int i = 0; i < kFramerate * 5; ++i) {

rtc_base/experiments/quality_scaling_experiment.cc

@@ -25,6 +25,11 @@ constexpr int kMaxVp9Qp = 255;
 constexpr int kMaxH264Qp = 51;
 constexpr int kMaxGenericQp = 255;
 
+#if !defined(WEBRTC_IOS)
+constexpr char kDefaultQualityScalingSetttings[] =
+    "Enabled-29,95,149,205,24,37,26,36,0.9995,0.9999,1";
+#endif
+
 absl::optional<VideoEncoder::QpThresholds> GetThresholds(int low,
                                                          int high,
                                                          int max) {
@@ -38,15 +43,22 @@ absl::optional<VideoEncoder::QpThresholds> GetThresholds(int low,
 }  // namespace
 
 bool QualityScalingExperiment::Enabled() {
+#if defined(WEBRTC_IOS)
   return webrtc::field_trial::IsEnabled(kFieldTrial);
+#else
+  return !webrtc::field_trial::IsDisabled(kFieldTrial);
+#endif
 }
 
 absl::optional<QualityScalingExperiment::Settings>
 QualityScalingExperiment::ParseSettings() {
-  const std::string group = webrtc::field_trial::FindFullName(kFieldTrial);
+  std::string group = webrtc::field_trial::FindFullName(kFieldTrial);
+  // TODO(http://crbug.com/webrtc/12401): Completely remove the experiment code
+  // after few releases.
+#if !defined(WEBRTC_IOS)
   if (group.empty())
-    return absl::nullopt;
+    group = kDefaultQualityScalingSetttings;
-
+#endif
   Settings s;
   if (sscanf(group.c_str(), "Enabled-%d,%d,%d,%d,%d,%d,%d,%d,%f,%f,%d",
              &s.vp8_low, &s.vp8_high, &s.vp9_low, &s.vp9_high, &s.h264_low,

rtc_base/experiments/quality_scaling_experiment_unittest.cc

@@ -38,10 +38,18 @@ void ExpectEqualConfig(QualityScalingExperiment::Config a,
 }
 }  // namespace
 
-TEST(QualityScalingExperimentTest, DisabledWithoutFieldTrial) {
+#if !defined(WEBRTC_IOS)
+// TODO(bugs.webrtc.org/12401): investigate why QualityScaler kicks in on iOS.
+TEST(QualityScalingExperimentTest, DefaultEnabledWithoutFieldTrial) {
+  webrtc::test::ScopedFieldTrials field_trials("");
+  EXPECT_TRUE(QualityScalingExperiment::Enabled());
+}
+#else
+TEST(QualityScalingExperimentTest, DefaultDisabledWithoutFieldTrialIOS) {
   webrtc::test::ScopedFieldTrials field_trials("");
   EXPECT_FALSE(QualityScalingExperiment::Enabled());
 }
+#endif
 
 TEST(QualityScalingExperimentTest, EnabledWithFieldTrial) {
   webrtc::test::ScopedFieldTrials field_trials(
@@ -59,10 +67,19 @@ TEST(QualityScalingExperimentTest, ParseSettings) {
   ExpectEqualSettings(kExpected, *settings);
 }
 
+#if !defined(WEBRTC_IOS)
+// TODO(bugs.webrtc.org/12401): investigate why QualityScaler kicks in on iOS.
+TEST(QualityScalingExperimentTest, ParseSettingsUsesDefaultsWithoutFieldTrial) {
+  webrtc::test::ScopedFieldTrials field_trials("");
+  // Uses some default hard coded values.
+  EXPECT_TRUE(QualityScalingExperiment::ParseSettings());
+}
+#else
 TEST(QualityScalingExperimentTest, ParseSettingsFailsWithoutFieldTrial) {
   webrtc::test::ScopedFieldTrials field_trials("");
   EXPECT_FALSE(QualityScalingExperiment::ParseSettings());
 }
+#endif
 
 TEST(QualityScalingExperimentTest, ParseSettingsFailsWithInvalidFieldTrial) {
   webrtc::test::ScopedFieldTrials field_trials(

video/video_stream_encoder_unittest.cc

@@ -5691,6 +5691,8 @@ TEST_F(VideoStreamEncoderTest, RampsUpInQualityWhenBwIsHigh) {
 
 TEST_F(VideoStreamEncoderTest,
        QualityScalerAdaptationsRemovedWhenQualityScalingDisabled) {
+  webrtc::test::ScopedFieldTrials field_trials(
+      "WebRTC-Video-QualityScaling/Disabled/");
   AdaptingFrameForwarder source(&time_controller_);
   source.set_adaptation_enabled(true);
   video_stream_encoder_->SetSource(&source,