Allow extensions to add, remove, or change their page action popup.

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 984 matching lines @@
               },
               "iconIndex": {
                 "type": "integer",
                 "minimum": 0,
                 "description": "<b>Deprecated.</b> The zero-based index into the <b>icons</b> vector specified in the manifest.",
                 "optional": true
               }
             }
           }
         ]
+      },
+      {
+        "name": "setPopup",
+        "type": "function",
+        "description": "Sets the html document to be opened as a popup when the user clicks on the page action's icon.",
+        "parameters": [
+          {
+            "name": "details",
+            "type": "object",
+            "properties": {
+              "tabId": {"type": "integer", "minimum": 0, "description": "The id of the tab for which you want to modify the page action."},
+              "popup": {
+                "type": "string",
+                "description": "The html file to show in a popup.  If set to the empty string (''), no popup is shown."
+              }
+            }
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onClicked",
         "type": "function",
-        "description": "Fired when a page action icon is clicked.",
+        "description": "Fired when a page action icon is clicked if the page action doesn't show a popup.",
         "parameters": [
           {
             "name": "tab",
             "$ref": "Tab"
           }
         ]
       }
     ]
   },
   {
@@ skipping 829 matching lines @@
         "type": "function",
         "description": "Logs a message during internal unit testing.",
         "parameters": [
           {"type": "string", "name": "message"}
         ]
       }
     ],
     "events": []
   }
 ]