win: Set is_clang=true by default.

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # WHAT IS THIS FILE?
 # =============================================================================
 #
 # This is the master GN build configuration. This file is loaded after the
 # build args (args.gn) for the build directory and after the toplevel ".gn"
@@ skipping 119 matching lines @@
   # separate flags.
   is_official_build = false
 
   # Whether we're a traditional desktop unix.
   is_desktop_linux = current_os == "linux"
 
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
   is_clang =
       current_os == "mac" || current_os == "ios" || current_os == "chromeos" ||
-      current_os == "fuchsia" ||
+      current_os == "fuchsia" || current_os == "win" ||
       (current_os == "linux" && current_cpu != "s390x" &&
        current_cpu != "s390" && current_cpu != "ppc64" && current_cpu != "ppc")
 
   # Allows the path to a custom target toolchain to be injected as a single
   # argument, and set as the default toolchain.
   custom_toolchain = ""
 
   # This should not normally be set as a build argument.  It's here so that
   # every toolchain can pass through the "global" value via toolchain_args().
   host_toolchain = ""
@@ skipping 539 matching lines @@
 set_defaults("component") {
   if (is_component_build) {
     configs = default_shared_library_configs
     if (is_android) {
       configs -= [ "//build/config/android:hide_all_but_jni_onload" ]
     }
   } else {
     configs = default_compiler_configs
   }
 }