Add instrumentation to help tracking issue 638400.

content/renderer/render_frame_impl.h

Index: content/renderer/render_frame_impl.h
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index 1f866b1748bd28b3c1d11af2af74d2881a56f31c..44277066d87c04c54ea536d548440609cf65a618 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -1267,6 +1267,9 @@ class CONTENT_EXPORT RenderFrameImpl
   // |pepper_last_mouse_event_target_| is not owned by this class. We depend on
   // the RenderFrameImpl to NULL it out when it destructs.
   PepperPluginInstanceImpl* pepper_last_mouse_event_target_;
+
+  // TODO(lfg): Temporary to help debug issue https://crbug.com/638400.
+  bool is_plugin_initializing_;
 #endif
 
   mojo::Binding<mojom::Frame> frame_binding_;