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This change replaces @available(macOS 10.10, *) with true and removes
API_AVAILABLE(macos(10.10)) annotations, then simplifies code where possible
as a result.

Bug: 841631
Change-Id: I55e0fbf23d836951354506f2058a419ebc95bb8b
Reviewed-on: https://chromium-review.googlesource.com/c/1437254Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626114}