Do not call StopSpeaking on TtsPlatform that is not available

R=dmazzoni@chromium.org

Fixed: 1113751
Change-Id: Ie7c04ab7b23b18ed6d980ed4b78fb6a756c59774
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341402Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795561}

content/browser/speech/tts_controller_impl.cc

@@ -170,7 +170,7 @@ bool TtsControllerImpl::StopCurrentUtteranceIfMatches(const GURL& source_url) {
   if (current_utterance_ && !current_utterance_->GetEngineId().empty()) {
     if (engine_delegate_)
       engine_delegate_->Stop(current_utterance_.get());
-  } else {
+  } else if (GetTtsPlatform()->PlatformImplAvailable()) {
     GetTtsPlatform()->ClearError();
     GetTtsPlatform()->StopSpeaking();
   }