[message_loop] Remove implicit cast from MessageLoopCurrent to MessageLoop*.

Remove implicit cast and convert relevant callsites to work around it.
Add hash, ostream and comparison operators to MLC.

TBR=gab@chromium.org
R=gab@chromium.org
CC=​​​​alexclarke@chromium.org
BUG=891670,826465

Change-Id: I04cda773496e1690160fb7b4236f6db9ab8c7d38
Reviewed-on: https://chromium-review.googlesource.com/c/1292886
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604354}
38 files changed