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Now that components/update_client/* have moved away from URLFetcher
and friends (see [1] and [2]), we can remove these helper classes.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1118646
[2] https://chromium-review.googlesource.com/c/chromium/src/+/1160725

BUG=844973

Change-Id: I362f135a913aba9bdddaa47a6c51a282c261a900
Reviewed-on: https://chromium-review.googlesource.com/1179941Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#584096}