Add onChanged events to the extension settings API, both from sync and between

chrome/test/data/extensions/api_test/settings/split_incognito/background.html

 <script>
 
 var settings = chrome.experimental.settings;
-
-// A different "waiting" message depending on whether this background page runs
-// in incognito mode or not.  This is so that the C++ test can differentiate
-// between messages.
+var assertEq = chrome.test.assertEq;
 
 // Returns a function that asserts a result is expected.
 function assertResultEq(expected) {
   return function(actual) {
     chrome.test.assertEq(expected, actual);
   };
 }
 
+// All notifications received.
+var notifications = [];
+settings.onChanged.addListener(function(changes) {
+  changes.forEach(function(change) {
+    notifications.push(change);
+  });
+});
+
 // The test from C++ runs "actions", where each action is defined here.
 // This allows the test to be tightly controlled between incognito and
 // non-incognito modes.
 var testActions = {
   noop: function() {
     settings.get("", function() {});
   },
   assertEmpty: function() {
     settings.get(null, assertResultEq({}));
   },
-  assertFoobar: function() {
+  assertFoo: function() {
     settings.get(null, assertResultEq({foo: "bar"}));
   },
-  setFoobar: function() {
+  setFoo: function() {
     settings.set({foo: "bar"}, assertResultEq({foo: "bar"}));
   },
   removeFoo: function() {
     settings.remove("foo", assertResultEq(undefined));
   },
   clear: function() {
     settings.clear(assertResultEq(undefined));
+  },
+  assertNoNotifications: function() {
+    assertEq([], notifications);
+  },
+  clearNotifications: function() {
+    notifications = [];
+  },
+  assertAddFooNotification: function() {
+    assertEq(1, notifications.length);
+    assertEq("foo", notifications[0].key);
+    assertEq(undefined, notifications[0].oldValue);
+    assertEq("bar", notifications[0].newValue);
+  },
+  assertDeleteFooNotification: function() {
+    assertEq(1, notifications.length);
+    assertEq("foo", notifications[0].key);
+    assertEq("bar", notifications[0].oldValue);
+    assertEq(undefined, notifications[0].newValue);
   }
 };
 
 // The only test we run.  Runs "actions" (as defined above) until told
 // to stop (when the message has isFinalAction set to true).
 function testEverything() {
   var waiting = chrome.extension.inIncognitoContext ?
       "waiting_incognito" : "waiting";
   function runTestAction(messageJson) {
     var message = JSON.parse(messageJson);
     testActions[message.action]();
     if (message.isFinalAction) {
       chrome.test.succeed();
       return;
     }
     chrome.test.sendMessage(waiting, runTestAction);
   }
   chrome.test.sendMessage(waiting, runTestAction);
 }
 
 chrome.test.runTests([testEverything]);
 
 </script>