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Until macOS 10.13, the relative ordering of a view's subviews' layers and
unassociated sublayers is undefined, so view layers can end up hidden behind
the compositor layer. See:

  https://developer.apple.com/library/archive/releasenotes/AppKit/RN-AppKit/index.html#10_13Layer-backed%20Views

This change re-adds a dedicated compositor view, but with less plumbing than
what was removed in r594160.

Bug: 865901, 899499
Change-Id: Ibbec83da2e8785e06522008dfc2d9eab7ca43bf9
Reviewed-on: https://chromium-review.googlesource.com/c/1334290
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608139}