Fix NULL dereference in ExtensionTabsModule.
Fix NULL dereference in ExtensionTabsModule.

This is almost certainly caused by a start-up or shut-down race, but CreateWindowValue doesn't have much meaning without a window attached to the browser, so just return an error in those cases

BUG=47808
TEST=none

Review URL: http://codereview.chromium.org/2885006

git-svn-id: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@51161 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser,wistoch/meego-app-browser