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It is only used within headless_content_main_delegate.cc, so do not
expose in the header.

Change-Id: If3b84d343906bc74683bee357ed3a3bb3748d67f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1927021
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717257}