GTK: minor MenuBarHelper fix.
GTK: minor MenuBarHelper fix.

If one of the buttons is not realized, gtk_widget_translate_coordinates() will fail on it. Instead of aborting, just skip to the next button. This can happen when opening a new window with a bookmark bar that doesn't have an overflow chevron (when the chevron has not been shown, it will be unrealized).

BUG=51810
TESt=see bug + above description

Review URL: http://codereview.chromium.org/3175006

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@55799 0039d316-1c4b-4281-b951-d872f2087c98

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