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This is part of the "always maximized" feature. Windows which cannot be maximized - or cannot be made to cover the entire screen should have a backdrop behind them which covers the desktop.

Tried various ways to implement this and this seems to be the best solution.

BUG=337567, 337563

Review URL: https://codereview.chromium.org/169643005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255863 0039d316-1c4b-4281-b951-d872f2087c98