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While the Blink tests for the speech synthesis API that we wrote use
internals to mock the speech engine, the speech synthesis Web Platform Tests
do not, so they were exercising real platform speech APIs on some platforms.
Fix that by providing a testing implementation of TtsPlatform specifically
for web tests.

This change doesn't try to get any new WPTs to pass, it just hopefully
introduces some more stability and consistency.

Bug: 1003410
Change-Id: I45972c4717978a52427fce06bfe2e236b67cfab3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1821741
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707439}