Removes WmRootWindowController subclasses

And makes mash use RootWindowController directly. My next step is to
fold WmRootWindowController back into RootWindowController.

BUG=671246
TEST=none
R=jamescook@chromium.org

Review-Url: https://codereview.chromium.org/2620913003
Cr-Commit-Position: refs/heads/master@{#442817}