Move enable extensions define to a build flag.

tools/json_schema_compiler/test/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 import("//build/json_schema_api.gni")
+import("//extensions/features/features.gni")
 import("//tools/json_schema_compiler/json_features.gni")
 
+assert(enable_extensions)
+
 json_schema_api("api") {
   visibility = [ ":*" ]
 
   sources = [
     "additional_properties.json",
     "any.json",
     "arrays.json",
     "callbacks.json",
     "choices.json",
     "crossref.json",
     "enums.json",
     "error_generation.json",
     "functions_as_parameters.json",
     "functions_on_types.json",
     "idl_basics.idl",
     "idl_object_types.idl",
     "idl_other_namespace.idl",
     "idl_other_namespace_sub_namespace.idl",
     "objects.json",
     "objects_movable.idl",
     "objects_movable_json.json",
     "simple_api.json",
   ]
 
   schemas = true
   root_namespace = "test::api::%(namespace)s"
+
+  deps = [
+    "//extensions/features",
+  ]
 }
 
-if (enable_extensions) {
-  json_features("features_compiler_test") {
-    feature_class = "APIFeature"
-    provider_class = "CompilerTestFeatureProvider"
-    sources = [
-      "features_test.json",
-      "features_test2.json",
-    ]
-  }
+json_features("features_compiler_test") {
+  feature_class = "APIFeature"
+  provider_class = "CompilerTestFeatureProvider"
+  sources = [
+    "features_test.json",
+    "features_test2.json",
+  ]
 }
 
-source_set("schema_test") {
+source_set("unit_tests") {
   testonly = true
   sources = [
+    "additional_properties_unittest.cc",
+    "any_unittest.cc",
+    "arrays_unittest.cc",
+    "callbacks_unittest.cc",
+    "choices_unittest.cc",
+    "crossref_unittest.cc",
+    "enums_unittest.cc",
+    "error_generation_unittest.cc",
+    "functions_as_parameters_unittest.cc",
+    "functions_on_types_unittest.cc",
+    "idl_schemas_unittest.cc",
+    "objects_unittest.cc",
+    "simple_api_unittest.cc",
     "test_util.cc",
     "test_util.h",
   ]
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 
   public_deps = [
+    ":features_compiler_test",
+  ]
+
+  deps = [
     ":api",
     "//base",
+    "//testing/gtest",
   ]
 }
-
-if (enable_extensions) {
-  source_set("features_generation_test") {
-    testonly = true
-    public_deps = [
-      ":features_compiler_test",
-    ]
-  }
-}