Work on GN Win64 build.

build/config/compiler/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/android/config.gni")
 if (cpu_arch == "arm") {
   import("//build/config/arm.gni")
 }
 if (cpu_arch == "mipsel" || cpu_arch == "mips64el") {
   import("//build/config/mips.gni")
@@ skipping 863 matching lines @@
 
 # This will generate warnings when using Clang if code generates exit-time
 # destructors, which will slow down closing the program.
 # TODO(thakis): Make this a blacklist instead, http://crbug.com/101600
 config("wexit_time_destructors") {
   if (is_clang) {
     cflags = [ "-Wexit-time-destructors" ]
   }
 }
 
+# On Windows compiling on x64, VC will issue a warning when converting
+# size_t to int because it will truncate the value. Our code should not have
+# these warnings and one should use a static_cast or a checked_cast for the
+# conversion depending on the case. However, a lot of code still needs to be
+# fixed. Apply this config to such targets to disable the warning.
+#
+# Note that this can be applied regardless of platform and architecture to
+# clean up the call sites. This will only apply the flag when necessary.
+config("no_size_t_to_int_warning") {
+  if (is_win && cpu_arch == "x64") {
+    cflags = [ "/wd4267" ]
+  }
+}
+
 # Optimization -----------------------------------------------------------------
 #
 # Note that BUILDCONFIG.gn sets up a variable "default_optimization_config"
 # which it will assign to the config it implicitly applies to every target. If
 # you want to override the optimization level for your target, remove this
 # config (which will expand differently for debug or release builds), and then
 # add back the one you want to override it with:
 #
 #   configs -= default_optimization_config
 #   configs += [ "//build/config/compiler/optimize_max" ]
@@ skipping 159 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }