Implement chrome.browserAction.setIcon({path:...});

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "Port",
         "type": "object",
         "description": "An object which allows two way communication with other pages.",
         "properties": {
           "name": {"type": "string"},
@@ skipping 841 matching lines @@
               "imageData": {
                 "type": "any",
                 "description": "Pixel data for an image. Must be an ImageData object (eg from a <code>canvas</code> element).",
                 "optional": true
               },
               "iconIndex": {
                 "type": "integer",
                 "minimum": 0,
                 "description": "The zero-based index into the |icons| vector specified in the manifest.",
                 "optional": true
+              },
+              "path": {
+                "type": "string",
+                "description": "Relative path to an image in the extension to show in the browser action.",
+                "optional": true
               }
             }
           }
         ]
       },
       {
         "name": "setBadgeText",
         "type": "function",
         "description": "Sets the badge text for the browser action. This is printed on top of the icon.",
         "parameters": [
@@ skipping 440 matching lines @@
         "type": "function",
         "description": "Logs a message during internal unit testing.",
         "parameters": [
           {"type": "string", "name": "message"}
         ]
       }
     ],
     "events": []
   }
 ]