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This change reduces the amount of code its complexity, and is the natural follow up from having alt tab separated into a different class.

Please note that no functionality has been changed, and that the change is pretty much only moving code around.

BUG=371884
TEST=WindowSelectorTest.*

Review URL: https://codereview.chromium.org/280423008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271142 0039d316-1c4b-4281-b951-d872f2087c98