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

chrome/common/extensions/api/windows.json

 [
   {
     "namespace": "windows",
     "types": [
       {
         "id": "Window",
         "type": "object",
         "properties": {
           "id": {"type": "integer", "minimum": 0, "description": "The ID of the window. Window IDs are unique within a browser session."},
           "focused": {"type": "boolean", "description": "Whether the window is currently the focused window."},
@@ skipping 14 matching lines @@
             "enum": ["normal", "minimized", "maximized"]
             }
         }
       }
     ],
     "properties": {
       "WINDOW_ID_NONE": {
         "type": "integer",
         "value": "-1",
         "description": "The windowId value that represents the absence of a chrome browser window."
+      },
+      "WINDOW_ID_CURRENT": {
+        "type": "integer",
+        "value": "-2",
+        "description": "The windowId value that represents the <a href='windows.html#current-window'>current window</a>."
       }
     },
     "functions": [
       {
         "name": "get",
         "type": "function",
         "description": "Gets details about a window.",
         "parameters": [
           {"type": "integer", "name": "windowId", "minimum": 0},
           {
@@ skipping 209 matching lines @@
         "name": "onFocusChanged",
         "type": "function",
         "description": "Fired when the currently focused window changes. Will be chrome.windows.WINDOW_ID_NONE if all chrome windows have lost focus. Note: On some Linux window managers, WINDOW_ID_NONE will always be sent immediately preceding a switch from one chrome window to another.",
         "parameters": [
           {"type": "integer", "name": "windowId", "minimum": -1, "description": "ID of the newly focused window."}
         ]
       }
     ]
   }
 ]