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This changes the currently unused V8ObjectVar to store a pointer to the
instance rather than a PP_Instance ID. This is just more convenient for
the code which uses the V8ObjectVar. When the instance is deleted, the
pointer will be cleared so it should always be valid or NULL.

BUG=351636

Review URL: https://codereview.chromium.org/464653003

Cr-Commit-Position: refs/heads/master@{#289191}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289191 0039d316-1c4b-4281-b951-d872f2087c98