overview: Introduce OverviewObserver.

Move overview ShellObserver methods to new class OverviewObserver.

Test: none
Bug: 914147
Change-Id: Ic0290fc9bd7e3144fc6939cc7833e008198a8ec1
Reviewed-on: https://chromium-review.googlesource.com/c/1461678
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631001}
28 files changed