Always finalize tts buffers on stop

This was left out (somewhat intentionally) in an effort to optimize the
init/read path. However, this appears to be occasionally causing the process to
hit resource limits on linux causing no new threads in the process to be
created.

Sample error:
2020-10-21T00:37:18.448831Z ERROR chrome[19371:19371]: [tts_service.cc(29)] E1020 17:37:18.448238   19371 thread.cc:68] Thread create failed: Resource temporarily unavailable

R=dmazzoni@chromium.org

Fixed: 1140617
Test: continually read long text over and over again. Observe no crash.
Change-Id: Ie811b6ecc7e2bce9f898dc7cbd351cdfe5e4503d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2489028
Auto-Submit: David Tseng <dtseng@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819265}

chromeos/services/tts/tts_service.cc

@@ -205,8 +205,10 @@ void TtsService::StopLocked(bool clear_buffers) {
     return;
 
   output_device_->Pause();
-  if (clear_buffers)
+  if (clear_buffers) {
     buffers_.clear();
+    libchrometts_.GoogleTtsFinalizeBuffered();
+  }
 
   is_playing_ = false;
 }