(not for commit) Simplified multi-threaded Ganesh with lock on Flush only.

cc/output/context_provider.h

Index: cc/output/context_provider.h
diff --git a/cc/output/context_provider.h b/cc/output/context_provider.h
index 7362e8b55aa9acffd62060af1ed08c766581c4f2..fbf1124670298d952b74909a627c05f07d8f8113 100644
--- a/cc/output/context_provider.h
+++ b/cc/output/context_provider.h
@@ -7,6 +7,7 @@
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
 #include "cc/base/cc_export.h"
 #include "gpu/command_buffer/common/capabilities.h"
 
@@ -27,6 +28,7 @@ class ContextProvider : public base::RefCountedThreadSafe<ContextProvider> {
   // Once this function has been called, the class should only be accessed
   // from the same thread.
   virtual bool BindToCurrentThread() = 0;
+  virtual void RebindToCurrentThread() {}
 
   virtual gpu::gles2::GLES2Interface* ContextGL() = 0;
   virtual gpu::ContextSupport* ContextSupport() = 0;