AudioRemdererMixer tests: replace unused ReturnMixer mock with actual implementation.

Bug: 696700
Change-Id: Id318bbb47c1392b175f5da2bed042b74cca3c111
Reviewed-on: https://chromium-review.googlesource.com/746642Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512855}

media/base/audio_renderer_mixer_unittest.cc

@@ -97,7 +97,9 @@ class AudioRendererMixerTest
     return mixer_.get();
   };
 
-  MOCK_METHOD1(ReturnMixer, void(AudioRendererMixer*));
+  void ReturnMixer(AudioRendererMixer* mixer) {
+    EXPECT_EQ(mixer_.get(), mixer);
+  }
 
   MOCK_METHOD4(
       GetOutputDeviceInfo,
@@ -515,7 +517,7 @@ TEST_P(AudioRendererMixerBehavioralTest, MixerPausesStream) {
 }
 
 INSTANTIATE_TEST_CASE_P(
-    AudioRendererMixerTest,
+    /* no prefix */,
     AudioRendererMixerTest,
     testing::Values(
         // No resampling, 1 input sample rate.