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FaviconBackend was using a mix of delegate and return values to
convey results. This was mildly confusing. This patch converts to
a return value. The one remaining delegate function is for redirects,
which makes sense as a delegate function.

BUG=1076463
TEST=covered by tests

Change-Id: I9bc84e9afcb02a417b62180cae5fe5a89a09ef28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357851
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798668}