[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /dbus.

This CL adds ScopedBlockingCall(MAY_BLOCK) for each scope containing a dbus_*
call, assuming all of those calls may block.
I kindly ask the reviewer to make sure of the following:
  - ScopedBlockingCall is instantiated in a scope with minimal CPU usage.
    If this is not the case, ScopedBlockingCall should be instantiated
    closer to the blocking call. See scoped_blocking_call.h for more
    info. Please let me know when/where the blocking call happens if this needs
    to be changed.
  - Parameter |blocking_type| matches expectation (MAY_BLOCK/WILL_BLOCK). See
    BlockingType for more info. While I assumed MAY_BLOCK by default, that might
    not be the best fit if we know that this callsite is guaranteed to block.
  - The ScopedBlockingCall's scope covers the entirety of the blocking operation
    previously asserted against by the AssertBlockingAllowed().

This CL was uploaded by git cl split.

R=satorux@chromium.org

Bug: 874080
Change-Id: Ib90b59280bd22cb671a64bacabe299c02c6d6081
Reviewed-on: https://chromium-review.googlesource.com/1190930
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#593177}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7f1f6062eb0057191403b8c14062d6aff4729ea8
2 files changed