WebUI: Fix compile_js2.py to retrigger compilation for <include> files.

chrome/browser/resources/print_preview/compiled_resources2.gyp

 # Copyright 2014 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.
 {
   'targets': [
     {
       'target_name': 'print_preview',
       'dependencies': [
         '<(DEPTH)/ui/webui/resources/js/compiled_resources2.gyp:cr',
         '<(DEPTH)/ui/webui/resources/js/compiled_resources2.gyp:event_tracker',
         '<(DEPTH)/ui/webui/resources/js/compiled_resources2.gyp:load_time_data',
         '<(DEPTH)/ui/webui/resources/js/compiled_resources2.gyp:promise_resolver',
         '<(DEPTH)/ui/webui/resources/js/compiled_resources2.gyp:util',
         '<(DEPTH)/ui/webui/resources/js/cr/compiled_resources2.gyp:event_target',
         '<(DEPTH)/ui/webui/resources/js/cr/ui/compiled_resources2.gyp:focus_manager',
         '<(DEPTH)/ui/webui/resources/js/cr/ui/compiled_resources2.gyp:focus_outline_manager',
         '<(DEPTH)/ui/webui/resources/js/cr/compiled_resources2.gyp:ui',
         '<(EXTERNS_GYP):chrome_send',
       ],
+      'variables': {
+        'extra_inputs': [
+          '<!@(python <(CLOSURE_DIR)/build/get_includes.py print_preview.js)',
+        ],
+      },
       'includes': ['../../../../third_party/closure_compiler/compile_js2.gypi'],
     }
   ],
 }