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This change adds a public ResultType alias to base::Callback, specifying
the return type of `Run()`. This is analogous to std::function's nested
result_type.

Bug: 554299
Change-Id: Ie478345b68051b03cd5b064b0833d528d47990b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436536
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812087}