Add __builtin_unreachable() to CRASH() when using Clang on Windows.

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")
 
 # 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",
     ]
 
     cflags = [
       # Don't complain about calling specific versions of templatized
       # functions (e.g. in RefPtrHashMap.h).
       "/wd4344",
       # dtoa, icu, etc. like doing assignment within conditional.
@@ skipping 103 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",
   ]
 }