Add support for a "frame" context option to chrome.contextMenus.create/update.

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 2816 matching lines @@
               "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"},
                 "minItems": 1,
                 "optional": true,
-                "description": "List of contexts this menu item will appear in. Legal values are: 'all', 'page', 'selection', 'link', 'editable', 'image', 'video', and 'audio'. Defaults to ['page']."
+                "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']."

[asargent_no_longer_on_chrome, 2011/04/06 20:39:33]

Some time after I first added the contextMenus API, we added an "enum"
capability but I forgot to convert over to using it here. If you want to it
would be cool to convert over to using an enum here instead of a string. That
way callers will get errors at call time from our dispatcher code if for
instance they misspell 'editable' or whatever, instead of having it just not
work right. 

[Mihai Parparita -not on Chrome, 2011/04/06 21:01:05]

Will do this in a follow-up CL.

               },
               "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 2114 matching lines @@
           {
             "type": "integer",
             "name": "tabId",
             "description": "The id of the tab that was detached."
           }
         ]
       }
     ]
   }
 ]