Fix crash that could happen when setting up caching of the wrapper for a document with no frame which was about to be deleted.
Fix crash that could happen when setting up caching of the wrapper for
a document with no frame which was about to be deleted.

The crash only happened occasionally when a JavaScript garbage
collection happened in the process of setting up the caching.  When
performing the garbage collection, the global handle to the document
might get cleared and the document deleted.

BUG=13780
Review URL: http://codereview.chromium.org/122032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@18157 0039d316-1c4b-4281-b951-d872f2087c98

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