Re-land: cc: Implement shared worker contexts.

cc/output/output_surface.h

Index: cc/output/output_surface.h
diff --git a/cc/output/output_surface.h b/cc/output/output_surface.h
index 9611ef4406e0fd18514c89a585d40f9f898085f2..ea68fa0f0dbacbc55ab89b389795425005c9015e 100644
--- a/cc/output/output_surface.h
+++ b/cc/output/output_surface.h
@@ -11,6 +11,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
 #include "cc/base/cc_export.h"
 #include "cc/output/context_provider.h"
 #include "cc/output/overlay_candidate_validator.h"
@@ -112,9 +113,14 @@ class CC_EXPORT OutputSurface {
 
   // Called by the compositor on the compositor thread. This is a place where
   // thread-specific data for the output surface can be initialized, since from
-  // this point on the output surface will only be used on the compositor
-  // thread.
+  // this point to when DetachFromClient() is called the output surface will
+  // only be used on the compositor thread.
   virtual bool BindToClient(OutputSurfaceClient* client);
+  // Called by the compositor on the compositor thread. This is a place where
+  // thread-specific data for the output surface can be uninitialized.
+  virtual void DetachFromClient();
+
+  bool CalledOnValidClientThread();
 
   virtual void EnsureBackbuffer();
   virtual void DiscardBackbuffer();
@@ -193,6 +199,7 @@ class CC_EXPORT OutputSurface {
 
  private:
   bool external_stencil_test_enabled_;
+  base::ThreadChecker client_thread_checker_;
 
   base::WeakPtrFactory<OutputSurface> weak_ptr_factory_;