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Having addressed all known stability issues on Mac, updates Mac enable
rate to be at parity with Windows.

Bug: 531673
Change-Id: I941d2a6d91fcc3272cdd64c98b1af6358301ea8a
Reviewed-on: https://chromium-review.googlesource.com/1101175Reviewed-by: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Mike Wittman <wittman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567418}