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This CL fixes an issue where the clipping window would get a focus event
when the SadTab was displayed causing the focus to be set to nothing,
disabling all keyboard shortcuts.

This is fixed by making the clipping window nonfocusable.

BUG=393119

Review URL: https://codereview.chromium.org/397493005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284177 0039d316-1c4b-4281-b951-d872f2087c98