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

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 987 matching lines @@
       },
       {
         "name": "onStop",
         "type": "function",
         "description": "Fired when a call is made to tts.stop and this extension may be in the middle of speaking. If an extension receives a call to onStop and speech is already stopped, it should do nothing (not raise an error)."
       }
     ]
   },
   {
     "namespace": "experimental.settings",
-    "types": [],
+    "types": [
+      {
+        "id": "SettingChange",
+        "type": "object",
+        "properties": {
+          "key": {"type": "string", "description": "The ID of the setting which changed."},
+          "oldValue": {
+            "type": "any",
+            "description": "The old value of the setting, before it changed.",
+            "optional": true
+          },
+          "newValue": {
+            "type": "any",
+            "description": "The new value of the setting.",
+            "optional": true
+          }
+        }
+      }
+    ],
     "functions": [
       {
         "name": "get",
         "type": "function",
         "description": "Gets one or more values from settings.",
         "parameters": [
           {
             "name": "keys",
             "choices": [
               {"type": "string"},
@@ skipping 77 matching lines @@
           {
             "name": "callback",
             "type": "function",
             "description": "Callback on success, or on failure (in which case lastError will be set).",
             "parameters": [],
             "optional": true
           }
         ]
       }
     ],
-    "events": []
+    "events": [
+      {
+        "name": "onChanged",
+        "type": "function",
+        "description": "Fired when one or more settings change.",
+        "parameters": [
+          {
+            "name": "changes",
+            "type": "array",
+            "items": {"$ref": "SettingChange"}
+          }
+        ]
+      }
+    ]
   },
   {
     "namespace": "windows",
     "types": [
       {
         "id": "Window",
         "type": "object",
         "properties": {
           "id": {"type": "integer", "minimum": 0, "description": "The ID of the window. Window IDs are unique within a browser session."},
           "focused": {"type": "boolean", "description": "Whether the window is currently the focused window."},
@@ skipping 7966 matching lines @@
                 "type": "array",
                 "items": { "$ref": "mimeTypeEnum" }
               }
             ]
           }
         ]
       }
     ]
   }
 ]