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

chrome/browser/resources/print_preview/compiled_resources2.gyp

Index: chrome/browser/resources/print_preview/compiled_resources2.gyp
diff --git a/chrome/browser/resources/print_preview/compiled_resources2.gyp b/chrome/browser/resources/print_preview/compiled_resources2.gyp
index a8b0dff44935474b936b9145257b0aaf72124244..34ec0aadb33fd45821635aa8018aa67f2024471a 100644
--- a/chrome/browser/resources/print_preview/compiled_resources2.gyp
+++ b/chrome/browser/resources/print_preview/compiled_resources2.gyp
@@ -17,6 +17,11 @@
         '<(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'],
     }
   ],