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Changes:
- Fix regex to accept very short otp code: #(.[]+) -> #([]+)
- Add tests to ensure we reject forbidden host characters
- Add additional tests around number of spaces expected

Followup patch will allow host to accept unicode.

Change-Id: I7cbeabe31edd9c9ead7ae78425c8760f9c78ce9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248071
Commit-Queue: Majid Valipour <majidvp@chromium.org>
Reviewed-by: Sam Goto <goto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780306}