Refactors Taskbar Decoration to make it accessible outside of views.

This change is in preparation for
https://chromium-review.googlesource.com/c/chromium/src/+/1429324

Bug: 719176
Change-Id: Ibb98c8fa087478a42de9868c372942919e3573fa
Reviewed-on: https://chromium-review.googlesource.com/c/1441971
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Jay Harris <harrisjay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627726}
9 files changed