Add low_bandwidth_audio_test to default build

https://codereview.webrtc.org/2717683002/ added a placeholder binary but
it still needs to be included in the default build before enabling it on
bots.

BUG=webrtc:7229

Review-Url: https://codereview.webrtc.org/2746403002
Cr-Commit-Position: refs/heads/master@{#17302}

webrtc/audio/BUILD.gn

@@ -78,6 +78,10 @@ if (rtc_include_tests) {
       "//testing/gmock",
       "//testing/gtest",
     ]
+    if (rtc_enable_protobuf) {
+      deps += [ ":low_bandwidth_audio_test" ]
+    }
+
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]