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The SerialPort.Open method originally took a SerialPortClient as an
associated interface. This ensures the ordering of responses to
SerialPort methods is synchronized with events on the SerialPortClient
interface.

This prevents SerialPort.Open from being used from the Mojo JS bindings
because they do not support associated interfaces. Web tests for the
Serial API implementation in Blink would like to use these bindings.

Upon deeper investigation the only method for which ordering really
matters is SerialPort.Open itself. This is because serial data is sent
using a pair of data pipes and those do not have any ordering guarantee
with either the SerialPort or SerialPortClient pipes.

To avoid messages on the now-unassociated SerialPortClient interface
from racing ahead of the reply to the SerialPort.Open method the
SerialConnection class is modified to only create its client binding
and start watchers on the data pipes after the reply to the Open method
is received.

Bug: 893334
Change-Id: I511060d45c5d537af1edddaeccd0436e34c0b0f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660603Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669578}