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: dd90618784b6a4b323ea0c23a071cb1c9e6f2ac7@37846 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