Break out WebAudio suspension code into new class. Add tests.

media/base/fake_audio_render_callback.cc

Index: media/base/fake_audio_render_callback.cc
diff --git a/media/base/fake_audio_render_callback.cc b/media/base/fake_audio_render_callback.cc
index fb686c56c14f05e0ce34da32602c1039eb005522..fc967c74fbd75bbb93fdbcd14f043598727984aa 100644
--- a/media/base/fake_audio_render_callback.cc
+++ b/media/base/fake_audio_render_callback.cc
@@ -36,13 +36,14 @@ int FakeAudioRenderCallback::Render(AudioBus* audio_bus,
 
   // Fill first channel with a sine wave.
   for (int i = 0; i < number_of_frames; ++i)
-    audio_bus->channel(0)[i] = sin(2 * M_PI * (x_ + step_ * i));
+    audio_bus->channel(0)[i] = sin(2 * M_PI * (x_ + step_ * i)) * volume_;
   x_ += number_of_frames * step_;
 
   // Copy first channel into the rest of the channels.
-  for (int i = 1; i < audio_bus->channels(); ++i)
+  for (int i = 1; i < audio_bus->channels(); ++i) {
     memcpy(audio_bus->channel(i), audio_bus->channel(0),
            number_of_frames * sizeof(*audio_bus->channel(i)));
+  }
 
   return number_of_frames;
 }