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

content/common/gpu/client/command_buffer_proxy_impl.cc

Index: content/common/gpu/client/command_buffer_proxy_impl.cc
diff --git a/content/common/gpu/client/command_buffer_proxy_impl.cc b/content/common/gpu/client/command_buffer_proxy_impl.cc
index cde985544c29754b5f8485c0c916486f4092a4d7..b6f78445bd2d8463629fc7dde7aa5412bb2b4096 100644
--- a/content/common/gpu/client/command_buffer_proxy_impl.cc
+++ b/content/common/gpu/client/command_buffer_proxy_impl.cc
@@ -434,6 +434,24 @@ bool CommandBufferProxyImpl::SignalSyncPoint(uint32 sync_point,
   return true;
 }
 
+bool CommandBufferProxyImpl::SignalQuery(unsigned query,
+                                         const base::Closure& callback) {
+  if (last_state_.error != gpu::error::kNoError) {
+    return false;
+  }
+
+  uint32 signal_id = next_signal_id_++;
+  if (!Send(new GpuCommandBufferMsg_SignalQuery(route_id_,
+                                                query,
+                                                signal_id))) {
+    return false;
+  }
+
+  signal_tasks_.insert(std::make_pair(signal_id, callback));
+
+  return true;
+}
+
 
 bool CommandBufferProxyImpl::GenerateMailboxNames(
     unsigned num,