Fix the problem of having a dangling layer_ pointer in RenderWidgetFullscreenPepper in some cases.
When PluginInstance calls Destroy() on fullscreen_container_ and resets the pointer, the corresponding RenderWidgetFullscreenPepper instance will post itself a Close message and live until that is processed. Before it actually goes away, if PluginInstance::UpdateLayer() is called, we may destroy web_layer_ without notifying the soon-to-die RenderWidgetFullscreenPepper instance to reset its layer_ pointer (because we have reset the pointer to it). This will result in a crash if a Resize message is processed before Close.
BUG=236473
TEST=None
Review URL: https://chromiumcodereview.appspot.com/
14960005
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@199200 0039d316-1c4b-4281-b951-d872f2087c98