Sync with chromium, primarily to shave the kOsFuchsia yak.

Bug: angleproject:2475
Change-Id: I76ce607095d02e805a41f63315b4237e57f1cc4e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1497392
Commit-Queue: Jamie Madill <jmadill@google.com>
Reviewed-by: Jamie Madill <jmadill@google.com>