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Prior to this CL, Cancel() returned a confusing boolean, which was
supposed to be used to indicate whether the `WebRtcEventLogUploaderImpl`
could be deleted directly, or if the caller to Cancel() was supposed to
wait for the completion callback to be posted back to it.

Instead, we now always wait for the completion callback to be
posted back.

Note that we also introduce a check to ensure the callback belongs
to the currently uploaded file, rather than to the previously
uploaded file.

Bug: 1092071
Change-Id: I7dd92d806bef540125bcafb49b57d763783c64eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260637
Commit-Queue: Elad Alon <eladalon@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781977}