Scheduler: Add support for tracing task origins

Add support for annotating posted scheduler tasks with a trace
location, i.e., the FROM_HERE macro. This makes it possible to identify
where a given task originated from while tracing.

BUG=391005

Review URL: https://codereview.chromium.org/399423004

git-svn-id: svn://svn.chromium.org/blink/trunk@178682 bbb929c8-8fbe-4397-9dbb-9b2b20218538