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Add implementation to chrome://.

Add implementation to chrome-untrusted://.

Add tests.

Bug: b:162051831
Change-Id: Ia0389cc9f7fbd052559121b8c69d521cc433777a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414178
Commit-Queue: Laurențiu Olteanu <lolteanu@google.com>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Mahmoud Gawad <mgawad@google.com>
Cr-Commit-Position: refs/heads/master@{#810172}