Add missing null handle check to document wrapper caching code.
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: http://src.chromium.org/svn/trunk/src@18697 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: dfbe4ef25886c6c355bd87b847026651348e918f
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser