Added an extra sync token field for extra command buffer identification.

mojo/gles2/command_buffer_client_impl.cc

Index: mojo/gles2/command_buffer_client_impl.cc
diff --git a/mojo/gles2/command_buffer_client_impl.cc b/mojo/gles2/command_buffer_client_impl.cc
index b9aa5028b298edf6538734fb148a33d2ac5fb533..9133fa87d132ab3ffad97679b88341bbe753e962 100644
--- a/mojo/gles2/command_buffer_client_impl.cc
+++ b/mojo/gles2/command_buffer_client_impl.cc
@@ -418,6 +418,10 @@ uint64_t CommandBufferClientImpl::GetCommandBufferID() const {
   return sync_client_impl_->GetCommandBufferID();
 }
 
+uint32_t CommandBufferClientImpl::GetExtraCommandBufferData() const {
+  return 0;
+}
+
 uint64_t CommandBufferClientImpl::GenerateFenceSyncRelease() {
   return next_fence_sync_release_++;
 }