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

content/renderer/media/audio_renderer_impl_unittest.cc

Index: content/renderer/media/audio_renderer_impl_unittest.cc
===================================================================
--- content/renderer/media/audio_renderer_impl_unittest.cc	(revision 118690)
+++ content/renderer/media/audio_renderer_impl_unittest.cc	(working copy)
@@ -87,8 +87,9 @@
     ASSERT_TRUE(channel_->Connect());
 
     mock_process_.reset(new MockRenderProcess);
+    // 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_);
 
     // Setup expectations for initialization.
     decoder_ = new media::MockAudioDecoder();