Delay UpdateRect until the SwapBuffers callback when accelerated compositing is on.

content/common/view_messages.h

Index: content/common/view_messages.h
diff --git a/content/common/view_messages.h b/content/common/view_messages.h
index d81829552acd0b7da0fa35f2a6dbd7f91622e962..6d72a39fe582321e1c0af310f883c189687579c6 100644
--- a/content/common/view_messages.h
+++ b/content/common/view_messages.h
@@ -518,6 +518,10 @@ IPC_STRUCT_BEGIN(ViewHostMsg_UpdateRect_Params)
   // which would indicate that this paint message is an ACK for multiple
   // request messages.
   IPC_STRUCT_MEMBER(int, flags)
+
+  // Whether or not the renderer expects a ViewMsg_UpdateRect_ACK for this
+  // update. True for 2D painting, but false for accelerated compositing.
+  IPC_STRUCT_MEMBER(bool, needs_ack)
 IPC_STRUCT_END()
 
 IPC_STRUCT_BEGIN(ViewHostMsg_DidFailProvisionalLoadWithError_Params)
@@ -1427,6 +1431,10 @@ IPC_MESSAGE_ROUTED2(ViewHostMsg_PaintAtSize_ACK,
 IPC_MESSAGE_ROUTED1(ViewHostMsg_UpdateRect,
                     ViewHostMsg_UpdateRect_Params)
 
+// Sent to unblock the browser's UI thread if it is waiting on an UpdateRect,
+// which may get delayed until the browser's UI unblocks.
+IPC_MESSAGE_ROUTED0(ViewHostMsg_UpdateIsDelayed)
+
 // Sent by the renderer when accelerated compositing is enabled or disabled to
 // notify the browser whether or not is should do painting.
 IPC_MESSAGE_ROUTED1(ViewHostMsg_DidActivateAcceleratedCompositing,