GitLab

Move the task of managing the task-runners for the worker-thread/main-thread in
a common implementation in WorkerThreadMessageFilter, and use that from the rest
of the message filters.

BUG=none
R=jochen@chromium.org, kinuko@chromium.org

Review URL: https://codereview.chromium.org/892543002

Cr-Commit-Position: refs/heads/master@{#314556}