Mojo C++ bindings: support mapping mojo string to WTF::String.

third_party/WebKit/Source/wtf/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("//testing/test.gni")
 import("//third_party/WebKit/Source/config.gni")
 
 # The list of files is kept in the .gypi.
 gypi_values = exec_script("//build/gypi_to_gn.py",
                           [ rebase_path("wtf.gypi") ],
                           "scope",
                           [ "wtf.gypi" ])
 
-visibility = [ "//third_party/WebKit/*" ]
+visibility = [
+  "//mojo/public/cpp/bindings/*",
+  "//third_party/WebKit/*",
+]
 
 config("wtf_config") {
   if (is_win) {
     defines = [
       "__STD_C",
       "_CRT_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_DEPRECATE",
     ]
     include_dirs = [ "os-win32" ]
 
@@ skipping 114 matching lines @@
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:non_test_config",
   ]
 
   deps = [
     ":wtf",
   ]
 }