Implement layerTreeId() in RenderWidgetCompositor.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index 6bc64532ed973b203244c0509ca35d3c9ea5d922..1835121df0d180053a0ccf3d88218f6cb70dce8e 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -741,6 +741,10 @@ void RenderWidgetCompositor::setDeferCommits(bool defer_commits) {
   layer_tree_host_->SetDeferCommits(defer_commits);
 }
 
+int RenderWidgetCompositor::layerTreeId() const {
+  return layer_tree_host_->id();
+}
+
 void RenderWidgetCompositor::setShowFPSCounter(bool show) {
   cc::LayerTreeDebugState debug_state = layer_tree_host_->debug_state();
   debug_state.show_fps_counter = show;