[Extensions] Add extension feature generation code

tools/json_schema_compiler/json_features.gypi

+# Copyright 2016 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.
+
+{
+  'variables': {
+    # When including this gypi, the following variables must be set:
+    #   in_files:
+    #     An array of json files from which to generate the feature provider.
+    #   out_dir:
+    #     The directory to put the generated code in under gen/.
+    #   out_base_filename:
+    #     The base filename to use for the generated feature provider; .h and
+    #     .cc will be appended.
+    #   feature_class:
+    #     The name of the class to use for features, e.g. APIFeature.
+    #   provider_class:
+    #     The name of the class to use for the feature provider, e.g.
+    #     APIFeatureProvider
+    'compiler_dir': '<(DEPTH)/tools/json_schema_compiler',
+    'compiler_script': '<(compiler_dir)/feature_compiler.py',
+  },
+  'actions': [
+    {
+      # GN version: json_features.gni
+      'action_name': 'genfeatures',
+      'msvs_external_rule': 1,
+      'extension': 'json',
+      'inputs': [
+        '<(compiler_dir)/code.py',
+        '<(compiler_script)',
+      ],
+      'outputs': [
+        '<(SHARED_INTERMEDIATE_DIR)/<(out_dir)/<(out_base_filename).cc',
+        '<(SHARED_INTERMEDIATE_DIR)/<(out_dir)/<(out_base_filename).h',
+      ],
+      'action': [
+        'python',
+        '<(compiler_script)',
+        '<(DEPTH)',
+        '<(feature_class)',
+        '<(provider_class)',
+        '<(SHARED_INTERMEDIATE_DIR)/<(out_dir)',
+        '<(out_base_filename)',
+        '<@(in_files)',
+      ],
+      'message': 'Generating C++ code for json feature files',
+      'process_outputs_as_sources': 1,
+    },
+  ],
+  'include_dirs': [
+    '<(SHARED_INTERMEDIATE_DIR)',
+    '<(DEPTH)',
+  ],
+  'direct_dependent_settings': {
+    'include_dirs': [
+      '<(SHARED_INTERMEDIATE_DIR)',
+    ]
+  },
+  # This target exports a hard dependency because it generates header
+  # files.
+  'hard_dependency': 1,
+}