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Replicating the equivalent BrowserThread documentation for ios, see
https://chromium-review.googlesource.com/c/chromium/src/+/1246361

Bug: 878356, 882872
Change-Id: Ie9470baecbce1dbb5b6c45852836db238cffe3b7
Reviewed-on: https://chromium-review.googlesource.com/1253763Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595418}