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This change adds an observer interface to the C++ and Mojo interfaces
for enumerating serial ports and wires it up to the serial port chooser
UI so that it updates in real time as devices are added and removed from
the system.

Bug: 918216
Change-Id: I9d0c3791a4dbb0093c1811d48b1a732bdf5c71fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094624
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751837}