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It has an implementation for ignored_by_shelf, so we don't need to
warn anymore.

Bug: none
Test: chrome --mash, don't see warning
Change-Id: Iee5a01ae50cc5a54190ce3dc6a1f71a23c980d83
Reviewed-on: https://chromium-review.googlesource.com/741864Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512602}