[Extensions] Add extension feature generation code

tools/json_schema_compiler/json_features.gni

+# 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.
+
+# Generates the FeatureProviders files for extension features files.
+# The following variables are required:
+#   sources: The features.json files to use.
+#   feature_class: The name of the feature class to generate, e.g. APIFeature.
+#   provider_class: The name of the provider class to generate, e.g.
+#                   APIFeatureProvider.
+template("json_features") {
+  assert(defined(invoker.sources),
+         "\"sources\" must be defined for the $target_name template.")
+  assert(defined(invoker.feature_class),
+         "\"feature_class\" must be defined for the $target_name template.")
+  assert(defined(invoker.provider_class),
+         "\"provider_class\" must be defined for the $target_name template.")
+  feature_class = invoker.feature_class
+  provider_class = invoker.provider_class
+
+  compiler_root = "//tools/json_schema_compiler"
+  target = "${target_name}_json_features"
+
+  action(target) {
+    sources = invoker.sources
+    script = "$compiler_root/feature_compiler.py"
+    inputs = []
+    outputs = [
+      "$target_gen_dir/$target_name.cc",
+      "$target_gen_dir/$target_name.h",
+    ]
+    rebased = rebase_path(invoker.sources, root_build_dir)
+    args = [
+             ".",
+             "$feature_class",
+             "$provider_class",
+             rebase_path(target_gen_dir, root_build_dir),
+             "$target",
+           ] + rebased
+  }
+
+  lib_sources = invoker.sources + get_target_outputs(":$target")
+  source_set(target_name) {
+    sources = lib_sources
+    public_deps = [
+      ":$target",
+    ]
+  }
+}