Removes a couple more functions from WmWindow

I started removing GetChildByShellWindowId(), but left it in places
that would have resulted in lots of unnecessary back and forth between
aura::Window and WmWindow.

BUG=687656
TEST=covered by tests
R=msw@chromium.org

Review-Url: https://codereview.chromium.org/2814243002
Cr-Commit-Position: refs/heads/master@{#464211}
33 files changed