Work on GN Win64 build.

Adds a new config to disable size_t -> int truncations and use it in various places where the x64 Windows build currently gives warnings.

This covers Chrome (except for Blink which has a separate patch) and the tests.

We should do a second pass to replace the existing instances of /wd4267 with this config.

Review URL: https://codereview.chromium.org/927363005

Cr-Original-Commit-Position: refs/heads/master@{#316874}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 1deb76bc64a51808cd3df93d8166f223241487fe

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"