gtk: Handle stale _NET_SUPPORTING_WM_CHECK properties.
gtk: Handle stale _NET_SUPPORTING_WM_CHECK properties.

We'd crash at startup if a window manager that supports
EWMH had been replaced during the current X session by
one that didn't, leaving behind a property on the root
window referring to an old window.

BUG=23860
TEST=checked that the old code crashes at startup and the new code doesn't after replacing openbox with xmonad

Review URL: http://codereview.chromium.org/568002

git-svn-id: http://src.chromium.org/svn/trunk/src@37846 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: b24633044c4c827719b1b30dfc98c654eda598fb
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