Don't create Pepper resources after the PepperPluginInstance's RenderFrame has gone away.

BUG=338984
R=bbudge@chromium.org

Review URL: https://codereview.chromium.org/135503003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249063 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249063