Migrate more base::MessageLoops to base::SingleThreadTaskExecutor

A large but trivial migration.

TBR=fdoray@chromium.org

Bug: 891670
Change-Id: I1d3a7575703de8ba17238cb1c9e64d1d825c08d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637460
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#666718}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f7fb8a8048304051ab3f5e7b5c89b2635ea8cf77
4 files changed