[exo] Register CustomFrameView as a WindowObserver.

Bug: 866370
Test: PIP window no longer has top portion cut off
Change-Id: I6a4415162fd2697b49866bbe8698d306315fe074
Reviewed-on: https://chromium-review.googlesource.com/1146524
Commit-Queue: Eliot Courtney <edcourtney@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577462}
2 files changed