[mus+ash] Removes WmWindow from ash RootWindowcontroller implementation

Moves WmWindow methods SetChildrenUseExtendedHitRegion and
ShouldUseExtendedHitRegion into window_util (as
SetChildrenUseExtendedHitRegionForWindow and
ShouldUseExtendedHitRegionForWindow) and adds kChildrenUseExtendedHitRegion
bool property that can be set on a container to use an easy targeter.

BUG=687656
TEST=None (refactoring)

Review-Url: https://codereview.chromium.org/2904273002
Cr-Commit-Position: refs/heads/master@{#475181}

• ash/mus/non_client_frame_controller.cc[diff]
• ash/root_window_controller.cc[diff]
• ash/root_window_controller.h[diff]
• ash/shelf/shelf_window_targeter.cc[diff]
• ash/shelf/shelf_window_targeter.h[diff]
• ash/wm/window_properties.cc[diff]
• ash/wm/window_properties.h[diff]
• ash/wm/window_util.cc[diff]
• ash/wm/window_util.h[diff]
• ash/wm_window.cc[diff]
• ash/wm_window.h[diff]