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All classes are now created/destroyed on the UI thread. The Content
Index Database internally deals with posting to/from the IO thread.

Bug: 973844
Change-Id: I05cd9bc20372f8d3c28180fd3f3b2d9402956e8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1749191
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688959}