Merge to 57: Consistent 30% improvement in audio mixer running time.

webrtc/modules/audio_mixer/audio_frame_manipulator.cc

Index: webrtc/modules/audio_mixer/audio_frame_manipulator.cc
diff --git a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
index ff4c31e6675d0642081878231864978b2e914323..8aa0b5ce02dc35c6bd3a9d4a90de48ca82f015d8 100644
--- a/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
+++ b/webrtc/modules/audio_mixer/audio_frame_manipulator.cc
@@ -29,6 +29,9 @@ void Ramp(float start_gain, float target_gain, AudioFrame* audio_frame) {
   RTC_DCHECK(audio_frame);
   RTC_DCHECK_GE(start_gain, 0.0f);
   RTC_DCHECK_GE(target_gain, 0.0f);
+  if (start_gain == target_gain) {
+    return;
+  }
 
   size_t samples = audio_frame->samples_per_channel_;
   RTC_DCHECK_LT(0, samples);