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

third_party/closure_compiler/compile_js2.gypi

 # Copyright 2015 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.
 {
   'type': 'none',
 
   'variables': {
     'CLOSURE_DIR': '<(DEPTH)/third_party/closure_compiler',
     'EXTERNS_GYP': '<(CLOSURE_DIR)/externs/compiled_resources2.gyp',
     'INTERFACES_GYP': '<(CLOSURE_DIR)/interfaces/compiled_resources2.gyp',
 
     'default_source_file': '<(_target_name).js',
     'source_files%': ['<(default_source_file)'],
+    'extra_inputs%': [],
 
     'includes': ['closure_args.gypi'],
   },
 
   'sources': ['<@(source_files)'],
 
   'all_dependent_settings': {
     'sources': ['<@(source_files)'],
   },
 
@@ skipping 23 matching lines @@
 
       'inputs': [
         '<(CLOSURE_DIR)/build/outputs.py',
         '<(CLOSURE_DIR)/closure_args.gypi',
         '<(CLOSURE_DIR)/compile2.py',
         '<(CLOSURE_DIR)/compile_js2.gypi',
         '<(CLOSURE_DIR)/compiler/compiler.jar',
         '<(CLOSURE_DIR)/include_js.gypi',
         '<(CLOSURE_DIR)/processor.py',
         '>@(_sources)',
+        # When converting to GN, write the paths to additional inputs in a GN
+        # depfile file instead.
+        '<@(extra_inputs)',
       ],
 
       'outputs': ['<(out_file)'],
 
       'action': [
         'python',
         '<(CLOSURE_DIR)/compile2.py',
         '<@(script_args)',
         '>@(_sources)',
         '--out_file', '<(out_file)',
         '--closure_args', '<@(closure_args)', '<@(disabled_closure_args)',
         # '--verbose' # for make glorious log spam of Closure compiler.
       ],
 
       'message': 'Compiling <(target_path)',
     },
   ],
 }