Work on GN Win64 build.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 164b97d26260f3e238834c36ffad1e9e53037f5c..584f72be2e30c6277aa04d6d005b788137e55d06 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -881,6 +881,20 @@ config("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"