Cleanup: Remove most enable_extensions logic in extensions code.

extensions/renderer/BUILD.gn

Index: extensions/renderer/BUILD.gn
diff --git a/extensions/renderer/BUILD.gn b/extensions/renderer/BUILD.gn
index ded00cf0b5a5a0e6c2373ae94d5ac27e2718a430..703ad5f74d7b98e5229d0feb3596a1dfd85614b7 100644
--- a/extensions/renderer/BUILD.gn
+++ b/extensions/renderer/BUILD.gn
@@ -2,6 +2,10 @@
 # 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")
+
+assert(enable_extensions)
+
 # GYP version: extensions/extensions.gyp:extensions_renderer
 source_set("renderer") {
   sources = [