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HeaderView is a sibling not child of the CustomFrameViewAsh, so
we need to set the visibility on overlay view as well.

BUG=b/110240289
TEST=manual, using following pip test app.
https://x20web.corp.google.com/users/ed/edcourtney/piptest.apk

Change-Id: Ide5fde4b9ead4fe89b349668d6cbce9ce7f49387
Reviewed-on: https://chromium-review.googlesource.com/1105624
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568360}