Use a mock WebAudioDevice for layout tests

content/renderer/renderer_webkitplatformsupport_impl.cc

Index: content/renderer/renderer_webkitplatformsupport_impl.cc
diff --git a/content/renderer/renderer_webkitplatformsupport_impl.cc b/content/renderer/renderer_webkitplatformsupport_impl.cc
index 8b13436e71395cc23adca7b5d147aaba6bc7b1e6..35c46b74cb8f6bf3f3e5a7886887fa914a672151 100644
--- a/content/renderer/renderer_webkitplatformsupport_impl.cc
+++ b/content/renderer/renderer_webkitplatformsupport_impl.cc
@@ -780,6 +780,12 @@ RendererWebKitPlatformSupportImpl::createAudioDevice(
     double sample_rate,
     WebAudioDevice::RenderCallback* callback,
     const WebKit::WebString& input_device_id) {
+  // Use a mock for testing.
+  WebKit::WebAudioDevice* mock_device =
+      GetContentClient()->renderer()->OverrideCreateAudioDevice(sample_rate);
+  if (mock_device)
+    return mock_device;
+
   // The |channels| does not exactly identify the channel layout of the
   // device. The switch statement below assigns a best guess to the channel
   // layout based on number of channels.