Fix mismatch about tabId in code and webrequest API documentation if requests don't belong to a tab

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 5400 matching lines @@
         "description": "Fired when a request is about to occur.",
         "parameters": [
           {
             "type": "object",
             "name": "details",
             "properties": {
               "requestId": {"type": "string", "description": "The ID of the request. Request IDs are unique within a browser session. As a result, they could be used to relate different events of the same request."},
               "url": {"type": "string"},
               "method": {"type": "string", "description": "Standard HTTP method."},
               "frameId": {"type": "integer", "description": "0 indicates the request happens in the main frame; a positive value indicates the ID of a subframe in which the request happens. If the document of a (sub-)frame is loaded (<code>type</code> is <code>main_frame</code> or <code>sub_frame</code>), <code>frameId</code> indicates the ID of this frame, not the ID of the outer frame. Frame IDs are unique within a tab."},
-              "tabId": {"type": "integer", "description": "The ID of the tab in which the request takes place. Set to null if the request isn't related to a tab."},
+              "tabId": {"type": "integer", "description": "The ID of the tab in which the request takes place. Set to -1 if the request isn't related to a tab."},
               "type": {"type": "string", "enum": ["main_frame", "sub_frame", "stylesheet", "script", "image", "object", "xmlhttprequest", "other"], "description": "How the requested resource will be used."},
               "timeStamp": {"type": "number", "description": "The time when the browser was about to make the request, in milliseconds since the epoch."}
             }
           }
         ],
         "extraParameters": [
           {
             "$ref": "RequestFilter",
             "optional": true,
             "name": "filter",
@@ skipping 2653 matching lines @@
                 "description": "A string result code. The value is non-empty on success only in tests.",
                 "optional": "true"
               }
             ]
           }
         ]
       }
     ]
   }
 ]