GitLab

I move creation to RootWindowControllerCommon, but I think
RootWindowControllerCommon should go away. I'm going to do that next,
which will result in some additional cleanup.

BUG=624173
TEST=covered by tests
R=msw@chromium.org

Review-Url: https://codereview.chromium.org/2292183003
Cr-Commit-Position: refs/heads/master@{#415374}