Cleanup: Remove most enable_extensions logic in chrome/ extensions code.

chrome/browser/extensions/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/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 
+assert(enable_extensions)
+
 gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("../../chrome_browser_extensions.gypi") ],
     "scope",
     [ "../../chrome_browser_extensions.gypi" ])
 
 # GYP version: chrome/chrome_browser_extensions.gypi:browser_extensions
 static_library("extensions") {
   sources = []
   sources += rebase_path(
@@ skipping 197 matching lines @@
         gypi_values.chrome_browser_extensions_app_list_sources,
         ".", "//chrome")
   }
 
   if (!use_ozone) {
     sources -= [
       "global_shortcut_listener_ozone.cc",
     ]
   }
 }