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AppKit runs its sheet animation in UI-thread blocking loop. There is no
way to swap in a frame from the compositor while it is running. So
failing to wait means we animate a blank window.

Fix by restoring the wait that regressed in r574771

Test=Bookmark Star -> More...: Ensure the window does not animate in blank.

Bug: 865331
Change-Id: Ifd87944e3ae0d3af5f5b7eb8569e9a7c98e1be0c
Reviewed-on: https://chromium-review.googlesource.com/1146136
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577186}