Add missing headers to fix chromium roll

Bug: None
Change-Id: If28819bbeebe739f07fcd8d6ea8ab841efc20f75
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176562
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31453}

audio/test/low_bandwidth_audio_test.cc

@@ -8,6 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include "absl/flags/declare.h"
 #include "absl/flags/flag.h"
 #include "api/test/simulated_network.h"
 #include "audio/test/audio_end_to_end_test.h"

audio/test/pc_low_bandwidth_audio_test.cc

@@ -10,7 +10,7 @@
 
 #include <memory>
 
-#include "absl/flags/flag.h"
+#include "absl/flags/declare.h"
 #include "api/test/create_network_emulation_manager.h"
 #include "api/test/create_peerconnection_quality_test_fixture.h"
 #include "api/test/network_emulation_manager.h"

test/testsupport/test_artifacts_unittest.cc

@@ -14,6 +14,7 @@
 
 #include <string>
 
+#include "absl/flags/declare.h"
 #include "absl/flags/flag.h"
 #include "rtc_base/system/file_wrapper.h"
 #include "test/gtest.h"