GTK: fix DCHECK triggered when multiple ExtensionPopupGtks existed concurrently.
GTK: fix DCHECK triggered when multiple ExtensionPopupGtks existed concurrently.

BUG=35211
TEST=see bug

Review URL: http://codereview.chromium.org/601014

git-svn-id: http://src.chromium.org/svn/trunk/src@38560 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: a9ba612f013f6cfcce1fc63d14afba198e7f6b23
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