Change ChildProcess::set_main_thread registration to occur inside the RenderThreadImpl::Init method.

content/test/webrtc_audio_device_test.cc

Index: content/test/webrtc_audio_device_test.cc
===================================================================
--- content/test/webrtc_audio_device_test.cc	(revision 118690)
+++ content/test/webrtc_audio_device_test.cc	(working copy)
@@ -121,8 +121,9 @@
                  base::Unretained(this), kThreadName));
   WaitForIOThreadCompletion();
 
+  // The RenderThreadImpl will register itself as the main thread for the
+  // current process.
   render_thread_ = new RenderThreadImpl(kThreadName);
-  mock_process_->set_main_thread(render_thread_);
 }
 
 void WebRTCAudioDeviceTest::TearDown() {