Update references to the extension messaging APIs to point to the "runtime"

chrome/common/extensions/docs/templates/intros/tts.html

 <table class="intro">
   <tr>
     <th scope="col"></th>
     <th scope="col"></th>
   </tr>
   <tr>
     <td><strong>Description:</strong></td>
     <td>Use the <code>chrome.tts</code> module to play synthesized
     text-to-speech (TTS).
     See also the related
@@ skipping 69 matching lines @@
 <p>To catch errors and make sure you're calling <code>speak()</code>
 correctly, pass a callback function that takes no arguments. Inside
 the callback, check
 $ref:runtime.lastError
 to see if there were any errors.</p>
 
 <pre>chrome.tts.speak(
     utterance,
     options,
     function() {
-      if (chrome.extension.lastError) {
-        console.log('Error: ' + chrome.extension.lastError.message);
+      if (chrome.runtime.lastError) {
+        console.log('Error: ' + chrome.runtime.lastError.message);
       }
     });</pre>
 
 <p>The callback returns right away, before the engine has started
 generating speech. The purpose of the callback is to alert you to
 syntax errors in your use of the TTS API, not to catch all possible
 errors that might occur in the process of synthesizing and outputting
 speech. To catch these errors too, you need to use an event listener,
 described below.</p>
 
@@ skipping 93 matching lines @@
 <pre>chrome.tts.getVoices(
     function(voices) {
       for (var i = 0; i < voices.length; i++) {
         console.log('Voice ' + i + ':');
         console.log('  name: ' + voices[i].voiceName);
         console.log('  lang: ' + voices[i].lang);
         console.log('  gender: ' + voices[i].gender);
         console.log('  extension id: ' + voices[i].extensionId);
         console.log('  event types: ' + voices[i].eventTypes);
       }
-    });</pre>
+    });</pre>