Uses a system-wide notion of isSpeaking in the Mac extension TTS api.

chrome/browser/speech/extension_api/tts_extension_api_platform.h

Index: chrome/browser/speech/extension_api/tts_extension_api_platform.h
diff --git a/chrome/browser/speech/extension_api/tts_extension_api_platform.h b/chrome/browser/speech/extension_api/tts_extension_api_platform.h
index 5fb90c511c9e48979b5246958b3050f691640590..038fbe05a574574aad08fe26a0b60ab668610702 100644
--- a/chrome/browser/speech/extension_api/tts_extension_api_platform.h
+++ b/chrome/browser/speech/extension_api/tts_extension_api_platform.h
@@ -34,6 +34,9 @@ class ExtensionTtsPlatformImpl {
   // Stop speaking immediately and return true on success.
   virtual bool StopSpeaking() = 0;
 
+  // Returns whether any speech is on going.
+  virtual bool IsSpeaking() = 0;
+
   // Return true if this platform implementation will fire the given event.
   // All platform implementations must fire the TTS_EVENT_END event at a
   // minimum.