Make chrome.tabs.update's tabId parameter optional.

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 1516 matching lines @@
               }
             ]
           }
         ]
       },
       {
         "name": "update",
         "type": "function",
         "description": "Modifies the properties of a tab. Properties that are not specified in <var>updateProperties</var> are not modified. Note: This function can be used without requesting the 'tabs' permission in the manifest.",
         "parameters": [
-          {"type": "integer", "name": "tabId", "minimum": 0},
+          {
+            "type": "integer",
+            "name": "tabId",
+            "minimum": 0,
+            "optional": true,
+            "description": "Defaults to the selected tab of the <a href='windows.html#current-window'>current window</a>."
+          },
           {
             "type": "object",
             "name": "updateProperties",
             "properties": {
               "url": {
                 "optional": true,
                 "description": "A URL to navigate the tab to."
               },
               "selected": {
                 "type": "boolean",
@@ skipping 6897 matching lines @@
             "type": "function",
             "description": "Called when the browser's passwords have been cleared.",
             "optional": true,
             "parameters": []
           }
         ]
       }
     ]
   }
 ]