Implement main blink target in GN.

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.
 
 # 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/*"
+
 config("wtf_config") {
   if (is_win) {
     defines = [
       "__STD_C",
       "_CRT_SECURE_NO_DEPRECATE",
       "_SCL_SECURE_NO_DEPRECATE",
       "CRASH=__debugbreak",
     ]
     include_dirs = [
       "os-win32",
@@ skipping 119 matching lines @@
   configs += [
     ":wtf_config",
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:non_test_config",
   ]
 
   deps = [
     ":wtf",
   ]
 }