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

android_webview/lib/main/aw_main_delegate.cc

Index: android_webview/lib/main/aw_main_delegate.cc
diff --git a/android_webview/lib/main/aw_main_delegate.cc b/android_webview/lib/main/aw_main_delegate.cc
index defb379388d38ac57af7a52be47e51c9e7262a70..ac2bac5f160a23c378d99215b0079e60670da088 100644
--- a/android_webview/lib/main/aw_main_delegate.cc
+++ b/android_webview/lib/main/aw_main_delegate.cc
@@ -90,6 +90,9 @@ bool AwMainDelegate::BasicStartupComplete(int* exit_code) {
   // WebView does not yet support screen orientation locking.
   cl->AppendSwitch(switches::kDisableScreenOrientationLock);
 
+  // WebView does not currently support Web Speech API (crbug.com/487255)
+  cl->AppendSwitch(switches::kDisableSpeechAPI);
+
   // WebView does not (yet) save Chromium data during shutdown, so add setting
   // for Chrome to aggressively persist DOM Storage to minimize data loss.
   // http://crbug.com/479767