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Rename CrossThreadPersistentRegion to CrossThreadPersistentRegions as we will be adding a second PersistentRegion for weak persistents:

BUG=757440

Change-Id: I751747470c5ddc1481270e181cdd1edcf9ae5291
Reviewed-on: https://chromium-review.googlesource.com/882626Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531830}