Perform glReadPixels with PBOs in the gpu, if PBOs are available.

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 763e657ebd5af9d3920c0ab8ff9d9eb1c11bccaf..f21cbb2ea3276029c8a3f6b2a2fcb11a1141a335 100644
--- a/content/common/gpu/gpu_messages.h
+++ b/content/common/gpu/gpu_messages.h
@@ -612,6 +612,13 @@ IPC_MESSAGE_ROUTED2(GpuCommandBufferMsg_SignalSyncPoint,
 IPC_MESSAGE_ROUTED1(GpuCommandBufferMsg_SignalSyncPointAck,
                     uint32 /* signal_id */)
 
+// Makes this command buffer signal when a query is reached, by sending
+// back a GpuCommandBufferMsg_SignalSyncPointAck message with the same
+// signal_id.
+IPC_MESSAGE_ROUTED2(GpuCommandBufferMsg_SignalQuery,
+                    uint32 /* query */,
+                    uint32 /* signal_id */)
+
 //------------------------------------------------------------------------------
 // Accelerated Video Decoder Messages
 // These messages are sent from Renderer process to GPU process.