Add BUILD.gn to build ui strings from grit.

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 27 matching lines @@
     "//third_party/leveldatabase",
     "//third_party/libpng",
     "//third_party/libusb",
     "//third_party/libwebp",
     "//third_party/re2",
     "//third_party/smhasher:cityhash",
     "//third_party/smhasher:murmurhash3",
     "//third_party/smhasher:pmurhash",
     "//third_party/zlib",
     "//tools/gn",
+    "//ui/base/strings",
     "//ui/events",
     "//ui/gfx",
     "//ui/resources",
     "//url",
     "//v8:v8_base",
   ]
 
   if (is_linux) {
     deps += [
       "//third_party/freetype2",
     ]
   }
 
   if (is_android) {
     deps -= [
       "//components/os_crypt",
       "//crypto",
       "//net",
       "//skia",
       "//third_party/libusb",
       "//tools/gn",
       "//ui/events",
       "//ui/gfx",
     ]
   }
 }