Move Local State's task runner to TaskScheduler.

The CrOS restart logic now has to reach into BrowserProcess to
ask for Local State to be flushed and get called back when it did.
There's quite a history there:
 * https://crbug.com/302578 is the reason EndSession() doesn't flush
   local_state() on CrOS.
 * But then it's explicitly needed on restarts (but still has to happen
   async so the kernel doesn't think the main thread is hung):
   https://codereview.chromium.org/6710001 + 
   https://codereview.chromium.org/6713032

R=robliao@chromium.org

Bug: 667892, 195612, chromium-os:13102, chromium-os:6924, 747495
Change-Id: I8fca8dba6a2f6c85b032367d78a26b1a92ae1b7a
Reviewed-on: https://chromium-review.googlesource.com/580336
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489871}
9 files changed