Cleanup: Remove most enable_extensions logic in extensions code.

extensions/renderer/BUILD.gn

 # Copyright 2014 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")
+
+assert(enable_extensions)
+
 # GYP version: extensions/extensions.gyp:extensions_renderer
 source_set("renderer") {
   sources = [
     "activity_log_converter_strategy.cc",
     "activity_log_converter_strategy.h",
     "api_activity_logger.cc",
     "api_activity_logger.h",
     "api_definitions_natives.cc",
     "api_definitions_natives.h",
     "app_runtime_custom_bindings.cc",
@@ skipping 146 matching lines @@
     "//skia",
     "//third_party/WebKit/public:blink",
   ]
 
   if (is_win) {
     cflags = [
       "/wd4267", # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     ]
   }
 }