Prevent extensions from clobbering JSON implementation that extension calls use

chrome/test/data/extensions/api_test/messaging/connect/page.js

+JSON.parse = function() {
+  return "JSON.parse clobbered by content script.";
+}
+
+JSON.stringify = function() {
+  return "JSON.stringify clobbered by content script.";
+}
+
+Array.prototype.toJSON = function() {
+  return "Array.prototype.toJSON clobbered by content script.";
+}
+
+Object.prototype.toJSON = function() {
+  return "Object.prototype.toJSON clobbered by content script.";
+}
+
 // For complex connect tests.
 chrome.extension.onConnect.addListener(function(port) {
   console.log('connected');
   port.onMessage.addListener(function(msg) {
     console.log('got ' + msg);
     if (msg.testPostMessage) {
       port.postMessage({success: true});
     } else if (msg.testPostMessageFromTab) {
       testPostMessageFromTab(port);
     } else if (msg.testSendRequestFromTab) {
@@ skipping 27 matching lines @@
       console.log('testSendRequestFromTab sent');
       chrome.extension.sendRequest({step: 2});
     }
   });
 }
 
 // For test sendRequest.
 chrome.extension.onRequest.addListener(function(request, sender, sendResponse) {
   sendResponse({success: (request.step2 == 1)});
 });