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This allows the urn: scheme in WebBundles' inner URLs, in addition to
http and https.

Bug: 1082020
Change-Id: I5582444e6d4f6ab29b83045a3d37a13f078071db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578300Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834605}