Make ui gn check pass. Misc GN build improvements.

ui/shell_dialogs/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/ui.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 component("shell_dialogs") {
@@ skipping 48 matching lines @@
     if (!is_android_webview_build) {
       #deps += [ '../android/ui_android.gyp:ui_java' ]  TODO(GYP)
     }
   }
 
   if (is_win) {
     #deps += [ '../../win8/win8.gyp:metro_viewer' ]  TODO(GYP)
   }
 }
 
-# TODO(GYP) enable shell_dialogs_unittests when media is converted.
-if (false) {
-
 test("shell_dialogs_unittests") {
   sources = [
     "select_file_dialog_win_unittest.cc",
   ]
 
   deps = [
     ":shell_dialogs",
     "//base",
     "//base/test:test_support",
     "//base/test:run_all_unittests",
     "//testing/gtest",
   ]
 }
-
-}  # if false