[scheduler] Support base::TaskTraits in blink's BackgroundScheduler

Add PostOnBackgrondThreadWithTraits to the blink's wrapper around
TaskScheduler and add relevant traits to the presubmit whitelist.

We want to allow Blink code to post background tasks with correct
traits but we still need a wrapper to ensure that CrossThreadBind
is used instead of base::Bind.

R=haraken@chromium.org
CC=​leszeks@chromium.org

Change-Id: If5297e1214bb175dee1247754158511811963b55
Reviewed-on: https://chromium-review.googlesource.com/1146658
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577509}
4 files changed