Adds Ash.WindowCycleController.Items UMA histogram that records how
many windows are included in Alt+Tab enumeration. The count is captured
when Alt+Tab is pressed.

Adds Ash.WindowCycleController.SelectionDepth UMA histogram that records
how often users finish Alt+Tab selection on a window that is N-th in MRU
order. Selecting a window that is already currently active (top in MRU
order) records 1.

BUG=621562

Review-Url: https://codereview.chromium.org/2096063002
Cr-Commit-Position: refs/heads/master@{#402365}