Add missing null handle check to document wrapper caching code.

If instantiation of the document wrapper fails (for instance in out of
memory situations) we get a null handle which will lead to a crash when
using ForceSet.

BUG=14514
Review URL: http://codereview.chromium.org/133001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@18697 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed