Move gtest/gmock BUILD.gn files to main tree.

build/config/win/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.
 
 import("//build/config/win/visual_studio_version.gni")
 
 # Compiler setup for the Windows SDK. Applied to all targets.
 config("sdk") {
   # The include path is the stuff returned by the script.
   #include_dirs = msvc_config[0]  TODO(brettw) make this work.
@@ skipping 91 matching lines @@
 }
 
 # Incremental linking ----------------------------------------------------------
 
 config("incremental_linking") {
   ldflags = [ "/INCREMENTAL" ]
 }
 config("no_incremental_linking") {
   ldflags = [ "/INCREMENTAL:NO" ]
 }
+
+# Disable unused variable warning ----------------------------------------------
+
+# Sometimes third-party code wants to disable this warning under MSVC, so this
+# config makes it easy to do so.
+config("disable_unused_variable_warning") {
+  cflags = [ "/wd4800" ]
+}