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This change builds on top of the logic to enqueue utterances for tts
platforms (which are interfaces to native tts api for that platform)
while initializing/loading. It adds the same treatment for
TtsEngineDelegate, which is responsible for built-in tts engines shipped
as extensions for each platform.

This starts off for built in engines on Chrome OS, but can encompass the
network tts engine we ship for other platforms. The new behavior will be
to enqueue until the built-in engine(s) are ready; readiness is signaled
by detecting at least one voice for the built-in engines
(Googletts/Espeak). Note it can be uninitialized in this scheme.

R=dmazzoni@chromium.org

Change-Id: Ie681e5830909245b566969b491043225c0640be1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552369
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829933}