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Some code (e.g. an app agent in follow-up CL) will require this header;
split it into a separate target to prevent circular deps.

Bug: none
Change-Id: I3c6c57f6e29bbf8c7156d30b71ba08681b2ecc93
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521124Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826832}