Guard some extensions build rules with enable_extensions.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 84 matching lines @@
 
   if (!is_ios && !is_android && !is_chromecast && !is_fuchsia) {
     deps += [
       "//chrome",
       "//chrome/test:browser_tests",
       "//chrome/test:interactive_ui_tests",
       "//chrome/test:sync_integration_tests",
       "//chrome/test/chromedriver:chromedriver_unittests",
       "//components/sync/tools:sync_client",
       "//components/sync/tools:sync_listen_notifications",
-      "//extensions:extensions_browsertests",
-      "//extensions:extensions_unittests",
       "//gpu/gles2_conform_support:gles2_conform_test",
       "//gpu/khronos_glcts_support:khronos_glcts_test",
       "//jingle:jingle_unittests",
       "//net:hpack_example_generator",
       "//ppapi:ppapi_unittests",
       "//ppapi/examples/2d",
       "//ppapi/examples/audio",
       "//ppapi/examples/audio_input",
       "//ppapi/examples/compositor",
       "//ppapi/examples/crxfs",
@@ skipping 83 matching lines @@
       "//url/ipc:url_ipc_unittests",
       "//v8:gn_all",
     ]
   } else if (is_ios) {
     deps += [ "//ios:all" ]
   }
 
   deps += root_extra_deps
 
   if (enable_extensions) {
-    deps += [ "//extensions/shell:app_shell_unittests" ]
+    deps += [
+      "//extensions:extensions_browsertests",
+      "//extensions:extensions_unittests",
+      "//extensions/shell:app_shell_unittests",
+    ]
   }
 
   if (enable_remoting) {
     deps += [ "//remoting:remoting_all" ]
   }
 
   if (toolkit_views) {
     deps += [
       "//ui/views:views_unittests",
       "//ui/views/examples:views_examples_exe",
@@ skipping 834 matching lines @@
   assert(target_name != "")  # Mark as used.
   sources = invoker.actual_sources
   assert(
       sources == invoker.actual_sources,
       "Do not use a platform name in your output directory (found \"$root_build_dir\"). http://crbug.com/548283")
 }
 
 assert_valid_out_dir("_unused") {
   actual_sources = [ "$root_build_dir/foo" ]
 }