Provide shared context to Platform API in renderer.

cc/context_provider.h

Index: cc/context_provider.h
diff --git a/cc/context_provider.h b/cc/context_provider.h
index ffb4822fe5d1c73842bc2d80b83f55251f1234f2..786d1470f1e35e65924b14a48a50fdf36a24bbba 100644
--- a/cc/context_provider.h
+++ b/cc/context_provider.h
@@ -32,6 +32,10 @@ class ContextProvider : public base::RefCountedThreadSafe<ContextProvider> {
   // one.
   virtual void VerifyContexts() = 0;
 
+  // A method to be called from the main thread that should return true if
+  // the context inside the provider is no longer valid.
+  virtual bool DestroyedOnMainThread() = 0;
+
  protected:
   friend class base::RefCountedThreadSafe<ContextProvider>;
   virtual ~ContextProvider() {}