Query the current window with chrome.tabs.query using -1.

chrome/common/extensions/api/tabs.json

 [
   {
     "namespace": "tabs",
     "types": [
       {
         "id": "Tab",
         "type": "object",
         "properties": {
           "id": {"type": "integer", "minimum": 0, "description": "The ID of the tab. Tab IDs are unique within a browser session."},
           "index": {"type": "integer", "minimum": 0, "description": "The zero-based index of the tab within its window."},
@@ skipping 238 matching lines @@
                 "description": "Match page titles against a pattern."
               },
               "url": {
                 "type": "string",
                 "optional": true,
                 "description": "Match tabs against a URL pattern."
               },
               "windowId": {
                 "type": "integer",
                 "optional": true,
-                "minimum": 0,
-                "description": "The ID of the parent window."
+                "minimum": -2,
+                "description": "The ID of the parent window, or <a href='windows.html#property-WINDOW_ID_CURRENT'>chrome.windows.WINDOW_ID_CURRENT</a> for the <a href='windows.html#current-window'>current window</a>."
               },
               "windowType": {
                 "type": "string",
                 "optional": true,
                 "enum": ["normal", "popup", "panel", "app"],
                 "description": "The type of window the tabs are in."
               }
             }
           },
           {
@@ skipping 486 matching lines @@
             "name": "removeInfo",
             "properties": {
               "isWindowClosing": {"type": "boolean", "description": "True when the tab is being closed because its window is being closed." }
             }
           }
         ]
       }
     ]
   }
 ]