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This change enhances the cancel() method on SerialPort.readable so that
is requests a flush of the serial port's receive buffer as required by
the draft specification[1].

[1]: https://reillyeon.github.io/serial/#dfn-serialportsourcecancelalgorithm

Bug: 989653
Change-Id: I8f6550ea6ac7093a04ca814b1f1b6b8d5d08ee76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2250366
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789677}