Move enable extensions define to a build flag.

extensions/features/features.gni

Index: extensions/features/features.gni
diff --git a/extensions/features/features.gni b/extensions/features/features.gni
new file mode 100644
index 0000000000000000000000000000000000000000..43da5b522e2ab142802f56dd5fc8e8b44c402fd1
--- /dev/null
+++ b/extensions/features/features.gni
@@ -0,0 +1,9 @@
+# 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/config/features.gni")
+
+declare_args() {
+  enable_extensions = !is_android && !is_ios
+}