Move enable extensions define to a build flag.

tools/json_schema_compiler/json_features.gni

Index: tools/json_schema_compiler/json_features.gni
diff --git a/tools/json_schema_compiler/json_features.gni b/tools/json_schema_compiler/json_features.gni
index cdf6c33a30a325550891f72d9e8fb7837cf58dce..ab940f72e52ef7cbd7f93ed9983eae5302116996 100644
--- a/tools/json_schema_compiler/json_features.gni
+++ b/tools/json_schema_compiler/json_features.gni
@@ -8,6 +8,7 @@
 #   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.
+#   deps/public_deps/visibility: normal meaning
 template("json_features") {
   assert(defined(invoker.sources),
          "\"sources\" must be defined for the $target_name template.")
@@ -21,12 +22,14 @@ template("json_features") {
   compiler_root = "//tools/json_schema_compiler"
   base_filename = target_name
   action_name = "${target_name}_json_features"
+  source_set_name = target_name
   generated_files = [
     "$target_gen_dir/$base_filename.cc",
     "$target_gen_dir/$base_filename.h",
   ]
 
   action(action_name) {
+    visibility = [ ":$source_set_name" ]
     sources = invoker.sources
     script = "$compiler_root/feature_compiler.py"
     inputs = [
@@ -42,12 +45,34 @@ template("json_features") {
              rebase_path(target_gen_dir, root_build_dir),
              "$base_filename",
            ] + rebased
+
+    # Add the deps in for the action as well, in case the deps generate the
+    # inputs used by the action.
+    forward_variables_from(invoker,
+                           [
+                             "deps",
+                             "public_deps",
+                           ])
+
+    # Append a dependency on the extensions system. Headers in this target
+    # are included by the feature compiler automatically.
+    if (!defined(deps)) {
+      deps = []
+    }
+    deps += [ "//extensions/common" ]
   }
 
   source_set(target_name) {
     sources = generated_files
-    public_deps = [
-      ":$action_name",
-    ]
+    forward_variables_from(invoker,
+                           [
+                             "deps",
+                             "public_deps",
+                             "visibility",
+                           ])
+    if (!defined(public_deps)) {
+      public_deps = []
+    }
+    public_deps += [ ":$action_name" ]
   }
 }