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This makes it clearer that WebRtcEventLogUploaderImpl runs on the
same task runner as WebRtcRemoteEventLogManager, and makes following
the threading issues easier.

Bug: 1092071
Change-Id: I0a72973525a03632b423dc69afc495439dfdaed5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260568
Commit-Queue: Elad Alon <eladalon@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781877}