Replace MessageLoop::current()->task_runner() with ThreadTaskRunnerHandle::Get().

This CL makes the following replacements:

MessageLoop::current()->task_runner()
becomes
ThreadTaskRunnerHandle::Get()

MessageLoopForUIorIO::current()->task_runner()
becomes
DCHECK(MessageLoopForUIorIO::IsCurrent()); (or ASSERT_TRUE)
ThreadTaskRunnerHandle::Get()

TBR=gavinp@chromium.org
BUG=650723

Review-Url: https://codereview.chromium.org/2375663002
Cr-Original-Commit-Position: refs/heads/master@{#422211}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 19e49e924e88e50546ec1a81dbc7b43ea4b172ef