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Cherry pick the latest hb_set improvements and optimizations to check
those against Chromium.

Bug: 813634
Change-Id: Idaba87ebf0811c6add23bc06737d94c0957cf3c3
Reviewed-on: https://chromium-review.googlesource.com/925521Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538103}