Introduce "disable-speech-api" content flag and add it to WebView.

content/child/runtime_features.cc

Index: content/child/runtime_features.cc
diff --git a/content/child/runtime_features.cc b/content/child/runtime_features.cc
index 7fae7425685ad68bc8661da9d64e9d71fb64cdda..c7a38b47868ea8aa3d792a138a29d441724c58ba 100644
--- a/content/child/runtime_features.cc
+++ b/content/child/runtime_features.cc
@@ -124,6 +124,9 @@ void SetRuntimeFeaturesDefaultsAndUpdateFromArgs(
     WebRuntimeFeatures::enableWebAudio(false);
 #endif
 
+  if (command_line.HasSwitch(switches::kDisableSpeechAPI))
+    WebRuntimeFeatures::enableScriptedSpeech(false);
+
   if (command_line.HasSwitch(switches::kDisableEncryptedMedia))
     WebRuntimeFeatures::enableEncryptedMedia(false);