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

content/common/gpu/client/command_buffer_proxy_impl.h

Index: content/common/gpu/client/command_buffer_proxy_impl.h
diff --git a/content/common/gpu/client/command_buffer_proxy_impl.h b/content/common/gpu/client/command_buffer_proxy_impl.h
index 7d04fdd436486c5c0bf9e2ea64681ab2e34067fa..f08c6e90136f08e6350d91e19d37bbc98287a500 100644
--- a/content/common/gpu/client/command_buffer_proxy_impl.h
+++ b/content/common/gpu/client/command_buffer_proxy_impl.h
@@ -114,6 +114,12 @@ class CommandBufferProxyImpl
   bool SignalSyncPoint(uint32 sync_point,
                        const base::Closure& callback);
 
+  // Makes this command buffer invoke a task when a query is completed, or
+  // the command buffer that inserted that sync point is destroyed.
+  // Should be invoked between begin/end query for reliable operation.
+  bool SignalQuery(unsigned query,
+                   const base::Closure& callback);
+
   // Generates n unique mailbox names that can be used with
   // GL_texture_mailbox_CHROMIUM. Unlike genMailboxCHROMIUM, this IPC is
   // handled only on the GPU process' IO thread, and so is not effectively