With this CL both the AsyncCompileJob and the ModuleCompiler use the new
TaskRunner API to post tasks. With the TaskRunner API it is also valid
to post foreground tasks from background tasks.

R=titzer@chromium.org

Change-Id: Ie3a1b0026f834c25540407eb79abdf67071915fb
Reviewed-on: https://chromium-review.googlesource.com/741590Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49419}