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The biggest change here is removing the idea of load success callbacks, which
are never used.

Bug: none
Change-Id: I31c1790e2c7e3bb1c3275327893ddee32533c808
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1962610
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Cait Phillips <caitkp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#727016}