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The current code is problematic because it's assuming
Window::SetEmbedFrameSinkId() synchronously calls
WindowPort::RegisterFrameSinkId(). That isn't necessarily the case as
Window calls RegisterFrameSinkId() when parented to a valid root. The fix
is to cache the FrameSinkId and compare against it.

BUG=none
TEST=none

Change-Id: Ib49b044921868ccca8c5feaba1d8a8716939bd28
Reviewed-on: https://chromium-review.googlesource.com/1240745Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593702}