Moves Add/RemoveShellObserver onto WmShell

Also adds the two OnOverviewMode calls to WmShell.

BUG=581868
TEST=none
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Review-Url: https://codereview.chromium.org/2051133002
Cr-Commit-Position: refs/heads/master@{#398934}
49 files changed