GitLab

- Do not allow assigning MessageLoop to MessageLoopCurrent.
- Remove hash operator from MessageLoopCurrent.

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

Change-Id: If558357328d1b07abaa5ccf7bcba5f9ef3609531
Reviewed-on: https://chromium-review.googlesource.com/c/1322874Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606033}