Refactors DimWindow and moves to ash/common

DimWindow was both an aura::Window and had some logic. As that won't
work at all in mash (and we generally don't subclass aura::Windows)
I've converted it to have a WmWindow. I also renamed it to
WindowDimmer.

BUG=631526
TEST=covered by tests
[email protected]

Review-Url: https://codereview.chromium.org/2320273002
Cr-Commit-Position: refs/heads/master@{#417755}
27 files changed