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As we no longer collect favicon availability metrics in FaviconSource,
we can remove the icon_request_origin member once added to the struct.
No behavior is changed.

Change-Id: I975d6f612a56be1e2fd7711dfa3b17e0db1fffba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643173Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/master@{#665989}