Fix re-entrant destroy issues in pepper
Fix re-entrant destroy issues in pepper

When the pepper plugin is called into, it has the ability to remove itself from
the DOM, using the Var interface.  To prevent the instance from going away while
it's still in the stack, we need to keep a reference to it, any time we call
into the plugin but need to access the interface afterwards.

BUG=chromium-os:11694
TEST=With pepper flash (needs separate fix), go to www.nick.com, let it play the video and check it doesn't crash

Review URL: http://codereview.chromium.org/6488017

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@74635 0039d316-1c4b-4281-b951-d872f2087c98

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