Limit panel support to a white list of window managers.
Limit panel support to a white list of window managers.

BUG=103449
TEST=Ran panel extension on multiple window managers. PanelBrowserTest.*.

Review URL: http://codereview.chromium.org/8501035

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@109166 0039d316-1c4b-4281-b951-d872f2087c98

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