Added Ash.NumberOfVisibleWindows UMA statistic to track the number of visible
windows in the primary root window.
In order to determine whether adding new window management would be useful we
want to know how many windows users generally interact with.
TEST=Manually verified the number of visible windows reported when
adding/removing/minimizing/maximizing/fullscreening windows in the different
containers.
BUG=470304
Review URL: https://codereview.chromium.org/
1047803003
Cr-Commit-Position: refs/heads/master@{#323369}