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See spec:
https://w3c.github.io/speech-api/#speechsynthesiserrorevent

and discussion on i2s on blink-dev:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/8_H3FUZm23g

Previously, errors were just going through as SpeechSynthesisEvents.
This CL now dispatches errors as SpeechSynthesisErrorEvents with an
empty "error" method.

This CL also adds constructors on both SpeechSynthesisEvent and
SpeechSynthesisErrorEvent.

Bug: 885297
Change-Id: I70ba998182943a50e17f2fc11eb7e163d8289e94
Reviewed-on: https://chromium-review.googlesource.com/1231839
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595128}