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

Index: chrome/common/extensions/api/extension_api.json
diff --git a/chrome/common/extensions/api/extension_api.json b/chrome/common/extensions/api/extension_api.json
index adb49126138d2bb806fab483b056a8825b159463..f948a0c0a6c8a4763abd310277b2e1bb18783bbb 100644
--- a/chrome/common/extensions/api/extension_api.json
+++ b/chrome/common/extensions/api/extension_api.json
@@ -5418,7 +5418,7 @@
               "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."}
             }