I just pass a pointer of the platform to the task runner so that the
task runners can put tasks directly into the platform data structures.

R=rmcilroy@chromium.org

Change-Id: I7c1c56c3b9f550c0fe80012abcd31011e69f3d5d
Reviewed-on: https://chromium-review.googlesource.com/771751Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49498}