cc: Make async readback path use async glRreadPixels.

cc/resources/resource_provider.h

Index: cc/resources/resource_provider.h
diff --git a/cc/resources/resource_provider.h b/cc/resources/resource_provider.h
index e964437757ba5ed4e6f7c8356bb083cc00c78e67..6f146d916ba64aa3a9cde12c015fece97e6b66a4 100644
--- a/cc/resources/resource_provider.h
+++ b/cc/resources/resource_provider.h
@@ -127,6 +127,10 @@ class CC_EXPORT ResourceProvider {
   // respect to other contexts.
   void Flush();
 
+  // Finish all context operations, causing any pending callbacks to be
+  // scheduled.
+  void Finish();
+
   // Only flush the command buffer if supported.
   // Returns true if the shallow flush occurred, false otherwise.
   bool ShallowFlushIfSupported();