Clean up error handling logic for extension tab muting

chrome/test/data/extensions/api_test/tabs/basics/muted.js

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 var testTabId_;
 
 chrome.test.runTests([
   function setupWindow() {
     chrome.tabs.getCurrent(pass(function(tab) {
       testTabId_ = tab.id;
@@ skipping 12 matching lines @@
       }));
     }));
   },
 
   function makeMuted() {
     onUpdatedExpect("muted", true, {mutedCause: chrome.runtime.id});
     chrome.tabs.update(testTabId_, {muted: true}, pass());
   },
 
   function testStaysMutedAfterChangingWindow() {
-    chrome.windows.create({}, pass(function(window)
-    {
+    chrome.windows.create({}, pass(function(window) {
       chrome.tabs.move(testTabId_, {windowId: window.id, index: -1},
                        pass(function(tab) {
         assertEq(true, tab.muted);
       }));
     }));
   },
 
   function makeNotMuted() {
     onUpdatedExpect("muted", false, {mutedCause: chrome.runtime.id});
     chrome.tabs.update(testTabId_, {muted: false}, pass());
+  },
+
+  function frequentRequestsRateLimited() {
+    chrome.tabs.update(testTabId_, {muted: true}, pass());

[miu, 2015/07/16 00:28:26]

Please add a comment at the top of this function that mentions prior tests in
this file have already toggled the mute state twice, and that this update() will
be the third (and final) time before rate limiting should kick in.

+    chrome.tabs.update(testTabId_, {muted: false}, function(tab) {
+      assertEq(true, tab.muted); // state not updated since rate limited

[miu, 2015/07/16 00:28:26]

For completeness, consider adding:

  assertTrue(!!chrome.runtime.lastError);
  if (chrome.runtime.lastError) {
    assertEq("<ERROR MESSAGE FOR RATE LIMITING HERE>",
chrome.runtime.lastError.message);
  }

+    });
   }
 ]);