Remove some more unnecessary thread-affinity in components/sync/

This replaces most remaining ThreadTaskRunnerHandle/SingleThreadTaskRunner
by SequencedTaskRunnerHandler/SequencedTaskRunner. There are some places
that actually need thread-affinity (e.g. network stuff), or where it makes
sense (UIModelWorker which is specifically for running on the UI thread).

One remaining place I'm not sure about is WeakHandle.

Bug: 846238
Change-Id: I704406fbf2c462537971eda0102c40626503b793
Reviewed-on: https://chromium-review.googlesource.com/1151296
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578312}
7 files changed