Add support for data deps.

tools/gn/gn.gyp

 {
   'variables': {
     'chromium_code': 1,
   },
   'targets': [
     {
       'target_name': 'gn_lib',
       'type': 'static_library',
       'dependencies': [
         '../../base/base.gyp:base',
@@ skipping 99 matching lines @@
         'tokenizer.cc',
         'tokenizer.h',
         'toolchain.cc',
         'toolchain.h',
         'toolchain_manager.cc',
         'toolchain_manager.h',
         'value.cc',
         'value.h',
         'value_extractors.cc',
         'value_extractors.h',
+        'variables.cc',
+        'variables.h',
       ],
     },
     {
       'target_name': 'gn',
       'type': 'executable',
       'sources': [
         'gn_main.cc',
       ],
       'dependencies': [
         'gn_lib',
@@ skipping 32 matching lines @@
       'type': 'executable',
       'sources': [
         'generate_test_gn_data.cc',
       ],
       'dependencies': [
         '../../base/base.gyp:base',
       ],
     }
   ],
 }