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This means that folks trying to navigate to chrome://help-app in a
browser will instead get the Help app opening as a standalone window.

Change-Id: I30941dc66f4c9528edfc0ead92d3d9551bc13d81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398604
Commit-Queue: Rachel Carpenter <carpenterr@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809669}