Mark mini_installer as chromium_code in gyp builds.

chrome/installer/mini_installer.gyp

 {
   'variables': {
+    'chromium_code': 1,
     'version_py': '<(DEPTH)/build/util/version.py',
     'version_path': '../../chrome/VERSION',
     'lastchange_path': '<(DEPTH)/build/util/LASTCHANGE',
     'branding_dir': '../app/theme/<(branding_path_component)',
     'msvs_use_common_release': 0,
     'msvs_use_common_linker_extras': 0,
     'mini_installer_internal_deps%': 0,
     'mini_installer_official_deps%': 0,
   },
   'includes': [
@@ skipping 33 matching lines @@
               'sources': [
                 'mini_installer_tests.isolate',
               ],
             },
           ],
         }],
       ],
     }],
   ],
 }