Add onChange event to preference extension APIs.

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 4631 matching lines @@
               },
               {
                 "name": "callback",
                 "type": "function",
                 "description": "Called after the preference has been cleared.",
                 "optional": true,
                 "parameters": []
               }
             ]
           }
+        ],
+        "events": [
+          {
+            "name": "onChange",
+            "description": "Fired when the value of the preference changes.",
+            "parameters": [
+              {
+                "type": "object",
+                "name": "details",
+                "properties": {
+                  "value": {
+                    "description": "The value of the preference.",
+                    "type": "any"
+                  },
+                  "levelOfControl": {
+                    "description": "One of<br><var>NotControllable</var>: cannot be controlled by any extension<br><var>ControlledByOtherExtensions</var>: controlled by extensions with higher precedence<br><var>ControllableByThisExtension</var>: can be controlled by this extension<br><var>ControlledByThisExtension</var>: controlled by this extension",
+                    "type": "string",
+                    "enum": ["NotControllable", "ControlledByOtherExtensions", "ControllableByThisExtension", "ControlledByThisExtension"]
+                  },
+                  "incognitoSpecific": {
+                    "description": "Whether the value that has changed is specific to the incognito session.<br>This property will <em>only</em> be present if the user has enabled the extension in incognito mode.",
+                    "type": "boolean",
+                    "optional": true
+                  }
+                }
+              }
+            ]
+          }
         ]
       }
     ]
   },
   {
     "namespace": "experimental.contentSettings.misc",
     "properties": {
       "blockThirdPartyCookies": {
         "$ref": "Preference",
         "value": ["blockThirdPartyCookies", {"type": "boolean"}],
         "description": "Whether third party cookies should be blocked. The value of this preference is of type boolean."
       }
     }
   }
 ]