Disallow speechSynthesis.speak autoplaying

See intent to remove:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/WsnBm53M4Pc

Even though speech API does not work properly on content shell, this
change can be tested in layout tests because it fails immediately without
calling into any synthesis code. To force autoplay, tests need to use
the new unified autoplay flag:
--autoplay-policy=document-user-activation-required

Bug: 812767
Change-Id: I41bee6e37ab46ff2013d096c714b5124bd0ccc2c
Reviewed-on: https://chromium-review.googlesource.com/1225650
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@{#595238}
6 files changed