This will allow base::PostTaskAndReplyWithResult() adapters to be
re-used from base/task_scheduler/post_task.h without indirectly
including the declaration of base::PostTaskAndReplyWithResult() in
every file that includes base/task_scheduler/post_task.h.

Note: task_runner_util.h will be renamed to
post_task_and_reply_with_result.h in a separate CL.

BUG=
TBR=sky@chromium.org,pkasting@chromium.org

Review-Url: https://codereview.chromium.org/2535843002
Cr-Commit-Position: refs/heads/master@{#435296}