Uncontroversial and easy to carve out bits of deprecated BrowserThreads cleanup.

Overall reland is at https://chromium-review.googlesource.com/c/chromium/src/+/705775,
these are the easy bits that can land first (and wrap up contributions to issues
365909 and 752144 as well as to the ios/ side of 689520).

NOPRESUBMIT=TRUE (for BrowserThread presubmit triggered by touching related code)
TBR=jam@chromium.org, jsbell@chromium.org, sdefresne@chromium.org, asvitkine@chromium.org

Bug: 689520, 365909, 752144
Change-Id: I9b2b5326a12de5f7ffa22f8ac3332c8e57a6e14f
Reviewed-on: https://chromium-review.googlesource.com/738469
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511551}
21 files changed