Change some context menu APIs to use enums in the schema.

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 2798 matching lines @@
           "type": "integer",
           "description": "The ID of the newly created item."
         },
         "parameters": [
           {
             "type": "object",
             "name": "createProperties",
             "properties": {
               "type": {
                 "type": "string",
+                "enum": ["normal", "checkbox", "radio", "separator"],
                 "optional": true,
-                "description": "The type of menu item - one of 'normal', 'checkbox', 'radio', or 'separator'. Defaults to 'normal' if not specified."
+                "description": "The type of menu item. Defaults to 'normal' if not specified."
               },
               "title": {
                 "type": "string",
-                "optional": "true",
+                "optional": true,
                 "description": "The text to be displayed in the item; this is <em>required</em> unless <em>type</em> is 'separator'. When the context is 'selection', you can use <code>%s</code> within the string to show the selected text. For example, if this parameter's value is \"Translate '%s' to Pig Latin\" and the user selects the word \"cool\", the context menu item for the selection is \"Translate 'cool' to Pig Latin\"."
               },
               "checked": {
                 "type": "boolean",
                 "optional": true,
                 "description": "The initial state of a checkbox or radio item: true for selected and false for unselected. Only one radio item can be selected at a time in a given group of radio items."
               },
               "contexts": {
                 "type": "array",
-                "items": {"type": "string"},
+                "items": {
+                  "type": "string",
+                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio"]
+                },
                 "minItems": 1,
                 "optional": true,
-                "description": "List of contexts this menu item will appear in. Legal values are: 'all', 'page', 'frame', 'selection', 'link', 'editable', 'image', 'video', and 'audio'. Defaults to ['page']."
+                "description": "List of contexts this menu item will appear in. Defaults to ['page'] if not specified."
               },
               "onclick": {
                 "type": "function",
                 "optional": true,
                 "description": "A function that will be called back when the menu item is clicked.",
                 "parameters": [
                   {
                     "name": "info",
                     "$ref": "OnClickData",
                     "description": "Information about the item clicked and the context where the click happened."
@@ skipping 47 matching lines @@
             "type": "object",
             "name": "updateProperties",
             "description": "The properties to update. Accepts the same values as the create function.",
             "properties": {
               "type": {
                 "type": "string",
                 "optional": true
               },
               "title": {
                 "type": "string",
-                "optional": "true"
+                "enum": ["normal", "checkbox", "radio", "separator"],
+                "optional": true
               },
               "checked": {
                 "type": "boolean",
                 "optional": true
               },
               "contexts": {
                 "type": "array",
-                "items": {"type": "string"},
+                "items": {
+                  "type": "string",
+                  "enum": ["all", "page", "frame", "selection", "link", "editable", "image", "video", "audio"]
+                },
                 "minItems": 1,
                 "optional": true
               },
               "onclick": {
                 "type": "function",
                 "optional": true
               },
               "parentId": {
                 "type": "integer",
                 "optional": true,
@@ skipping 2038 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]