Mark Surface as damaged when it receives a CopyOutputRequest.

This forces the Display containing it to redraw and satisfy the request.

BUG=

Review URL: https://codereview.chromium.org/582173003

Cr-Commit-Position: refs/heads/master@{#296018}

cc/surfaces/surface_factory.cc

@@ -54,6 +54,7 @@ void SurfaceFactory::RequestCopyOfSurface(
   }
   DCHECK(it->second->factory() == this);
   it->second->RequestCopyOfOutput(copy_request.Pass());
+  manager_->SurfaceModified(surface_id);
 }
 
 void SurfaceFactory::ReceiveFromChild(