Update existing uses of /wd4267 to use the GN config

This is the size_t to integer conversion warning. Using the config prevents flag duplication and is clearer to read.

Minor updates to Win64 build, including some fixes for size_t to int conversions.

Review URL: https://codereview.chromium.org/929793006

Cr-Original-Commit-Position: refs/heads/master@{#317162}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d1c719acf7db459016f425eb8018ec4208511578
diff --git a/BUILD.gn b/BUILD.gn
index cd9b453..3c5215e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -148,8 +148,6 @@
     # re2c is missing CLOSEVOP from one switch.
     if (is_posix) {
       cflags = [ "-Wno-switch" ]
-    } else if (is_win) {
-      cflags = [ "/wd4267" ]  # size_t to int conversion.
     }
   }
 
@@ -248,9 +246,7 @@
     # directory, but the gen_x86_insn.py script does not make this easy.
     include_dirs = [ yasm_gen_include_dir ]
 
-    if (is_win) {
-      cflags = [ "/wd4267" ]  # size_t to int conversion.
-    } else {
+    if (!is_win) {
       cflags = [
         "-ansi",
         "-pedantic",