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This disables window manager borders, and supports dragging of the
skyline, as well as resizing from the corners.

This patch also moves our ::Atom caching out of RootWindowHostLinux and into its own Singleton.

BUG=125106
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10381063

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137525 0039d316-1c4b-4281-b951-d872f2087c98