[serial] Finish wiring up serial guard permission

This change finishes wiring up the CONTENT_SETTINGS_TYPES_SERIAL_GUARD
permission which can be used to entirely block the chooser UI shown by
navigator.serial.requestPort(). As with the USB guard permission this
needs special casing in the page info and site settings UIs because it
can't be set to "allow," only "ask" or "block."

Bug: 908836
Change-Id: I085c9e60e8a3cd6fa85ca0b18a58f359ef35e7f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538681
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652697}
14 files changed