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This change adds the beginnings of the Serial API[1] to Blink. The
functions on navigator.serial and SerialPort are unimplemented.

[1]: https://wicg.github.io/serial

Bug: 884928
Change-Id: I76b26337570b07ecf9b265c2876c516058e90ee8
Reviewed-on: https://chromium-review.googlesource.com/1238486Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594052}