This CL switches to the new taskrunner API in the MemoryReducer. We
want to remove the old API eventually so that there is only one API for
task posting.

R=ulan@chromium.org
CC=gab@chromium.org

Change-Id: I1dd16e42dc0fca013a97c76dfce9c6479b97521b
Reviewed-on: https://chromium-review.googlesource.com/1061531Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53422}