AGC test shouldn't enable AEC.

The test
WebRtcAudioQualityBrowserTest.MANUAL_TestAutoGainControlOnLowAudio
sets up a WebRTC audio-only one-way connection between two Chrome
tabs. The first tab receives recorded audio from a fake microphone and
sends it to the second tab. The second tab plays the audio which is
then recorded.

The WebRTC connection is negotiated with constraints "{audio: true}",
which enable the AEC. The AEC affects audio levels, and changes in it
potentially cause alerts such as this one:
https://chromeperf.appspot.com/group_report?bug_id=779057

On some test settings (with '--single-process'), the AEC of one tab
sees the playout stream (reverse) of the other tab, causing it to
cancel the 'echo'. That leads to heavy speech distortion and often
test failure (see the bug chromium:779057).

This change disables AEC in the AGC tests.

Bug: chromium:779057
Change-Id: Ie774e2faba41f057e47fb2ed4ee402217da8aaf3
Reviewed-on: https://chromium-review.googlesource.com/746641Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Commit-Queue: Alex Loiko <aleloi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513028}

chrome/browser/media/webrtc/webrtc_audio_quality_browsertest.cc

@@ -835,8 +835,11 @@ void MAYBE_WebRtcAudioQualityBrowserTest::TestAutoGainControl(
 IN_PROC_BROWSER_TEST_F(MAYBE_WebRtcAudioQualityBrowserTest,
                        MANUAL_TestAutoGainControlOnLowAudio) {
   base::ScopedAllowBlockingForTesting allow_blocking;
+  // Disables AEC, but leaves AGC on.
+  const char* kAudioCallWithoutEchoCancellation =
+      "{audio: { mandatory: { googEchoCancellation: false } } }";
   ASSERT_NO_FATAL_FAILURE(TestAutoGainControl(
-      kReferenceFile, kAudioOnlyCallConstraints, "_with_agc"));
+      kReferenceFile, kAudioCallWithoutEchoCancellation, "_with_agc"));
 }
 
 // Since the AGC is off here there should be no gain at all.