Enable media stream layout test with content_shell.

content/public/renderer/content_renderer_client.cc

Index: content/public/renderer/content_renderer_client.cc
===================================================================
--- content/public/renderer/content_renderer_client.cc	(revision 191534)
+++ content/public/renderer/content_renderer_client.cc	(working copy)
@@ -53,6 +53,16 @@
   return NULL;
 }
 
+WebKit::WebUserMediaClient*
+ContentRendererClient::OverrideUserMediaClient() {
+  return NULL;
+}
+
+webkit_media::MediaStreamClient*
+ContentRendererClient::OverrideMediaStreamClient() {
+  return NULL;
+}
+
 WebKit::WebRTCPeerConnectionHandler*
 ContentRendererClient::OverrideCreateWebRTCPeerConnectionHandler(
     WebKit::WebRTCPeerConnectionHandlerClient* client) {