Add a manifest property declaring USB printers supported by an app.

extensions/common/manifest_constants.h

 // Copyright 2013 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 EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_
 #define EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_
 
 namespace extensions {
 
 // Keys used in JSON representation of extensions.
@@ skipping 157 matching lines @@
 extern const char kTtsVoicesEventTypes[];
 extern const char kTtsVoicesGender[];
 extern const char kTtsVoicesLang[];
 extern const char kTtsVoicesRemote[];
 extern const char kTtsVoicesVoiceName[];
 extern const char kType[];
 extern const char kUIOverride[];
 extern const char kUpdateURL[];
 extern const char kUrlHandlers[];
 extern const char kUrlHandlerTitle[];
+extern const char kUsbPrinters[];
 extern const char kVersion[];
 extern const char kVersionName[];
 extern const char kWebAccessibleResources[];
 extern const char kWebURLs[];
 extern const char kWebview[];
 extern const char kWebviewName[];
 extern const char kWebviewAccessibleResources[];
 extern const char kWebviewPartitions[];
 extern const char kWhitelist[];
 
@@ skipping 288 matching lines @@
 extern const char kWebRequestConflictsWithLazyBackground[];
 #if defined(OS_CHROMEOS)
 extern const char kIllegalPlugins[];
 #endif
 
 }  // namespace manifest_errors
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_MANIFEST_CONSTANTS_H_