gtk: Don't crash if the user double clicks an empty area of the task manager tree view.
gtk: Don't crash if the user double clicks an empty area of the task manager tree view.

BUG=18960
TEST=Double click the empty space of the task manager tree view.  The browser should not crash.
Review URL: http://codereview.chromium.org/165253

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@22942 0039d316-1c4b-4281-b951-d872f2087c98

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