Remove SpeechSynthesis runtime flag (status=stable)

public/web/WebRuntimeFeatures.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 82 matching lines @@
     BLINK_EXPORT static void enableRequestAutocomplete(bool);
 
     BLINK_EXPORT static void enableScreenOrientation(bool);
 
     BLINK_EXPORT static void enableScriptedSpeech(bool);
 
     BLINK_EXPORT static void enableServiceWorker(bool);
 
     BLINK_EXPORT static void enableSessionStorage(bool);
 
-    BLINK_EXPORT static void enableSpeechSynthesis(bool);
-
     BLINK_EXPORT static void enableTouch(bool);
 
     BLINK_EXPORT static void enableTouchIconLoading(bool);
 
     BLINK_EXPORT static void enableWebAnimationsSVG(bool);
 
     BLINK_EXPORT static void enableWebAudio(bool);
 
     BLINK_EXPORT static void enableWebGLDraftExtensions(bool);
 
@@ skipping 19 matching lines @@
 
     BLINK_EXPORT static void enableLaxMixedContentChecking(bool);
 
 private:
     WebRuntimeFeatures();
 };
 
 } // namespace blink
 
 #endif