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This CL is moving the speech synthesizer code to a worker thread.

The TtsPlatformImpl is using an ISpVoice COM interface to perform
text-to-speech actions. The instance was living on the UI Main
Thread which was using STA Apartment.

This CL is splitting the logic in two parts. The first part is the
control logic which stay on the UI Thread. The second part is the
synthesizer logic which is running on a worker thread.

Bug: 869399, 1133813
Change-Id: I8f8d939950cc39637f87d101ed362123a51e985e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462341
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820340}