Fix getUserMedia so that failure is reported for invalid audio sources.

content/shell/renderer/layout_test/blink_test_runner.cc

Index: content/shell/renderer/layout_test/blink_test_runner.cc
diff --git a/content/shell/renderer/layout_test/blink_test_runner.cc b/content/shell/renderer/layout_test/blink_test_runner.cc
index 155cff00948f4133540634676bb2e876af421217..cf81c8e824fc76a7492fa8f181940f94d0fe7590 100644
--- a/content/shell/renderer/layout_test/blink_test_runner.cc
+++ b/content/shell/renderer/layout_test/blink_test_runner.cc
@@ -208,14 +208,24 @@ class MockAudioCapturerSource : public media::AudioCapturerSource {
 
   void Initialize(const media::AudioParameters& params,
                   CaptureCallback* callback,
-                  int session_id) override {}
-  void Start() override {}
+                  int session_id) override {
+    callback_ = callback;
+  }
+  void Start() override {
+    if (callback_)
+      callback_->OnCaptureStarted();
+  }
   void Stop() override {}
   void SetVolume(double volume) override {}
   void SetAutomaticGainControl(bool enable) override {}
 
  protected:
   ~MockAudioCapturerSource() override {}
+
+ private:
+  CaptureCallback* callback_ = nullptr;
+
+  DISALLOW_COPY_AND_ASSIGN(MockAudioCapturerSource);
 };
 
 // Tests in web-platform-tests use absolute path links such as