Move ParallelJobs to filters and rewrite it on top of blink threads

To plan is to move those filters to SkiaImageFilters at which point
we can delete ParallelJobs.

BUG=301537
R=abarth@chromium.org,senorblanco@chromium.org,jamesr@chromium.org

Review URL: https://codereview.chromium.org/68513010

git-svn-id: svn://svn.chromium.org/blink/trunk@161937 bbb929c8-8fbe-4397-9dbb-9b2b20218538