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Consolidate the frame-sink registration in one place in aura (Window),
for aura. It used to be spread between aura (WindowPort*), and content
(DelegatedFrameHost), which made it tricky to follow. This then makes
it easier to move all FrameSinkId management inside the aura::Window
(from the various WindowPort implementations).

BUG=none

Change-Id: I37943eace983a31485dc6c0587d90619781b7021
Reviewed-on: https://chromium-review.googlesource.com/935490Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539253}