Add support for a "split" incognito behavior for extensions.

chrome/test/data/extensions/api_test/incognito/apis/background.html

 <script>
 var normalWindow, normalTab;
 var incognitoWindow, incognitoTab;
 
 var pass = chrome.test.callbackPass;
 var assertEq = chrome.test.assertEq;
 var assertTrue = chrome.test.assertTrue;
 
 chrome.test.runTests([
   function setupWindows() {
@@ skipping 58 matching lines @@
           pass(function(tab) {
             assertEq(incognitoTab.incognito, tab.incognito);
             chrome.tabs.remove(tab.id, pass());
           }));
       }));
   },
 
   // Tests content script injection to verify that the script can tell its
   // in incongnito.
   function contentScriptTestIncognito() {
-    // The property is undefined outside of content scripts.
-    assertEq(undefined, chrome.extension.inIncognitoTab);
+    assertTrue(!chrome.extension.inIncognitoContext);
 
     var testUrl = "http://localhost:1337/files/extensions/test_file.html";
 
     // Test that chrome.extension.inIncognitoTab is true for incognito tabs.
     chrome.tabs.create({windowId: incognitoWindow.id, url: testUrl},
       pass(function(tab) {
         chrome.tabs.executeScript(tab.id,
-          {code: 'document.title = chrome.extension.inIncognitoTab'},
+          {code: 'document.title = chrome.extension.inIncognitoContext'},
           pass(function() {
             assertEq(undefined, chrome.extension.lastError);
             chrome.tabs.get(tab.id, pass(function(tab) {
                 assertEq("true", tab.title);
               }));
           }));
       }));
 
     // ... and false for normal tabs.
     chrome.tabs.create({windowId: normalWindow.id, url: testUrl},
       pass(function(tab) {
         chrome.tabs.executeScript(tab.id,
-          {code: 'document.title = chrome.extension.inIncognitoTab'},
+          {code: 'document.title = chrome.extension.inIncognitoContext'},
           pass(function() {
             assertEq(undefined, chrome.extension.lastError);
             chrome.tabs.get(tab.id, pass(function(tab) {
                 assertEq("false", tab.title);
               }));
           }));
       }));
   }
 ]);
 </script>