Eliminate Shell::GetWmRootWindowForNewWindows().
Convert ShellPort::Get*RootWindow*() to aura::Window.
(use Shell::GetPrimaryRootWindow() instead in many places)
Convert ScopedRootWindowForNewWindows (etc.) to aura::Window.
Convert WmShelf::ForWindow to take aura::Window.
Cleanup some helpers and misc WmWindow usage.

BUG=722551,687656
TEST=Automated; no behavior changes
R=jamescook@chromium.org,xiyuan@chromium.org

Review-Url: https://codereview.chromium.org/2886253002
Cr-Commit-Position: refs/heads/master@{#472934}