Drop frontbuffers with ui-use-gpu-process, synchronized with browser, decoupled from backbuffer dro…

content/common/gpu/gpu_messages.h

Index: content/common/gpu/gpu_messages.h
diff --git a/content/common/gpu/gpu_messages.h b/content/common/gpu/gpu_messages.h
index bd6ce9449d60d511573fdb9280edcbc1fe3d600b..7aa86218ea85668bc9140746b407103d9640bff3 100644
--- a/content/common/gpu/gpu_messages.h
+++ b/content/common/gpu/gpu_messages.h
@@ -192,6 +192,10 @@ IPC_MESSAGE_ROUTED2(AcceleratedSurfaceMsg_NewACK,
                     uint64 /* surface_handle */,
                     TransportDIB::Handle /* shared memory buffer */)
 
+// Tells the GPU process that it's safe to release the surface.
+IPC_MESSAGE_ROUTED2(AcceleratedSurfaceMsg_ReleaseACK,
+                    uint64 /* surface_id */, bool /* success */)
+
 // Tells the GPU process that the browser process handled the swap
 // buffers request.
 IPC_MESSAGE_ROUTED0(AcceleratedSurfaceMsg_BuffersSwappedACK)