GitLab

Split all IO thread code into a separate helper class. Now there is no
refcounting, so ownership is clearer. Now only the helper class has a
WeakPtrFactory, so WeakPtrs are only used on the IO thread.

BUG=873529

Change-Id: I326aa6f69839c193507df4c63250aa1560bbd0ef
Reviewed-on: https://chromium-review.googlesource.com/1177444
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583813}