At the moment, task runners are returned as unique_ptr. This is
inconvenient, however. In all implementations I did, the platform holds
a shared pointer of the task runner and wraps it in a wrapper class just
to return it as a unique_ptr. With this CL the platform API is changed
to return a shared_ptr directly.

R=rmcilroy@chromium.org

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ide278db855199ea239ad0ae14d97fd17349dac8c
Reviewed-on: https://chromium-review.googlesource.com/768867
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49366}