[mus+ash] Removes WmWindow from ash/wm/focus_rules

With this CL WmWindow is no longer used in ash/wm.

BUG=722553
TEST=None (refactoring)

Review-Url: https://codereview.chromium.org/2908643003
Cr-Commit-Position: refs/heads/master@{#475121}
11 files changed