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

chrome/common/extensions/extension_constants.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_
 #define CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_
 #pragma once
 
 #include <string>
 
@@ skipping 310 matching lines @@
 
   // The file to write our decoded images to, relative to the extension_path.
   extern const char kDecodedImagesFilename[];
 
   // The file to write our decoded message catalogs to, relative to the
   // extension_path.
   extern const char kDecodedMessageCatalogsFilename[];
 }
 
 namespace extension_misc {
+  // Matches chrome.windows.WINDOW_ID_NONE.
   const int kUnknownWindowId = -1;
 
+  // Matches chrome.windows.WINDOW_ID_CURRENT.
+  const int kCurrentWindowId = -2;
+
   // The extension id of the bookmark manager.
   extern const char kBookmarkManagerId[];
 
   // The extension id of the Web Store component application.
   extern const char kWebStoreAppId[];
 
   // The extension id of the Cloud Print component application.
   extern const char kCloudPrintAppId[];
 
   // Note: this structure is an ASN.1 which encodes the algorithm used
@@ skipping 111 matching lines @@
 
   enum UnloadedExtensionReason {
     UNLOAD_REASON_DISABLE,    // Extension is being disabled.
     UNLOAD_REASON_UPDATE,     // Extension is being updated to a newer version.
     UNLOAD_REASON_UNINSTALL,  // Extension is being uninstalled.
     UNLOAD_REASON_TERMINATE,  // Extension has terminated.
   };
 }  // extension_misc
 
 #endif  // CHROME_COMMON_EXTENSIONS_EXTENSION_CONSTANTS_H_