Return scoped_refptr in ThreadScheduler::*TaskRunner().

This is a purely cosmetic change -- the behavior remains the same.

Currently, WebThreadScheduler::*TaskRunner() returns a scoped_refptr,
while ThreadScheduler::*TaskRunner() returns a raw pointer. This incon-
sistency causes a small trouble in a later patch, so this patch lets
those functions always return scoped_refptr.

Bug: 826203
Change-Id: Ie448c8bc39d395c27a2565a762222bb38e63c7e2
Reviewed-on: https://chromium-review.googlesource.com/1056894
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558254}
9 files changed