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Also convert net::FileStreamContext to use it, as a sample use case with
few consumers.

This CL does not add a OnceCallback version of TestCompletionCallback,
as the repeating version works for use with methods that take
OnceCallbacks.  I think not creating a new test class will make updating
interfaces take a lot less effort.

Also, I expect that all net::CompletionCallbacks are effectively used
as OnceCallbacks, so we can make TestCompletionCallback return
OnceCallbacks when/if we ever finish converting all of net to use
CompletionOnceCallbacks.

Bug: 714018
Change-Id: Iec76deae075b32fe0c65eb18596859f5e6413647
Reviewed-on: https://chromium-review.googlesource.com/853092
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Randy Smith <rdsmith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533061}