[message_loop] Remove message_loop_forward.h

As a final step, replace all includes back:
- mv message_loop_current.h message_loop.h
- s/message_loop_forward.h/message_loop.h/ in all includes.
- s/message_loop_forward.h/message_loop.h/ in base/BUILD.gn
- Remove message_loop_forward.h from third_party/DEPS.

TBR=gab@chromium.org
R=gab@chromium.org
BUG=891670

Change-Id: I623077025701459ddb7045cbcfdad138aa90a9e4
Reviewed-on: https://chromium-review.googlesource.com/c/1313110
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#604690}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4f9c35c3635144e2c2d1b87d09cbcf076d138b66
11 files changed