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- Replace 2 instances of WeakPtr usage in DialServiceImpl with a
CancelableTaskTracker to track thread-hopping calls to obtain network
interfaces.
- Replace the callbacks bound to DialServiceImpl using WeakPtr passed
to DialSocket with a raw pointer DialServiceImpl. This is safe because
DialServiceImpl owns DialSockets.
- Remove WeakPtrFactory from DialServiceImpl.

Bug: 735590
Change-Id: I4965a33a7a706b057ee7d0f4c5cb6fb98b06dbc2
Reviewed-on: https://chromium-review.googlesource.com/574993Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Derek Cheng <imcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487595}