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- Use a base::AutoReset instance to manage a single integer tracking
  whether the completion callback for an operation is invoked before
  we have actually returned to the caller.
- Remove the now-unused OperationHandle and BeginOperationScoper.
- Replace SupportsWeakPtr with an internal WeakPtrFactory.
- Replace potentially unsafe use of GetWeakPtr() with a |weak_ptr_|
  member.
- Temporarily disables the recently-added FileSystemURLLoaderFactoryTest
  tests, which have incorrect threading, causing WeakPtr checks to
  fire.

Bug: 846985, 860547
Change-Id: Ia059b1b87ef11f3218aeb6c65d7b8dd62be6c393
Reviewed-on: https://chromium-review.googlesource.com/1074427
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Chris Mumford <cmumford@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574220}