Move enable extensions define to a build flag.

extensions/features/BUILD.gn

Index: extensions/features/BUILD.gn
diff --git a/extensions/features/BUILD.gn b/extensions/features/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..5be8600f6a9271597f7e0126773e015e21cde4b4
--- /dev/null
+++ b/extensions/features/BUILD.gn
@@ -0,0 +1,16 @@
+# 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.
+
+import("//build/buildflag_header.gni")
+import("//extensions/features/features.gni")
+
+# This file is in a separate directory so all targets in the build can refer to
+# the buildflag header to get the necessary preprocessor defines without
+# bringing in all of extensions. Other targets can depend on this target
+# regardless of whether extensions are enabled.
+
+buildflag_header("features") {
+  header = "features.h"
+  flags = [ "ENABLE_EXTENSIONS=$enable_extensions" ]
+}