use root window for properties on tab-drag controller

Some properties are used to share the status between ash and
TabDragController; they are synchronized by crrev.com/584167,
but actually it isn't enough. Since ash expects these properties
are set on windows backed by Widget -- which is corresponded with
the root window in browser, TabDragController needs to set, clear,
and get the properties on root-window too instead of the widget.

Bug: 867074
Test: manually
Change-Id: I501db87283c66a6d5c80f92aa4b333d93d0ae415
Reviewed-on: https://chromium-review.googlesource.com/1182409
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584843}
1 file changed