Fix webrtcAudioPrivate API to handle requests from <webview>s.

content/public/test/mock_render_process_host.h

Index: content/public/test/mock_render_process_host.h
diff --git a/content/public/test/mock_render_process_host.h b/content/public/test/mock_render_process_host.h
index 326f2af1a0e1ef4d4f6248fccf959e1506788126..328625999d85f493dbe4ea67fee2fb216aeacccb 100644
--- a/content/public/test/mock_render_process_host.h
+++ b/content/public/test/mock_render_process_host.h
@@ -112,6 +112,9 @@ class MockRenderProcessHost : public RenderProcessHost {
     process_handle = new_handle.Pass();
   }
 
+  void GetAudioOutputControllers(
+      const GetAudioOutputControllersCallback& callback) const override {}
+
  private:
   // Stores IPC messages that would have been sent to the renderer.
   IPC::TestSink sink_;