GitLab

MessageLoopImpl was removed by crrev.com/c/1440828, but there are still
somce forward declarations around.

Change-Id: I39d1e18632b97aa6a7aa2e59fca61db997651034
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974817
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726676}