Update from https://crrev.com/317530

third_party/yasm/BUILD.gn

 # Copyright 2014 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.
 
 # The yasm build process creates a slew of small C subprograms that
 # dynamically generate files at various point in the build process.  This makes
 # the build integration moderately complex.
 #
 # There are three classes of dynamically generated files:
 #   1) C source files that should be included in the build (eg., lc3bid.c)
@@ skipping 12 matching lines @@
 #              shared between yasm and the genperf subprogram.
 #   4) genmacro, genmodule, etc. -- One executable target for each subprogram.
 #   5) generate_license, generate_module, etc. -- Actions that invoke programs
 #              built in #4 to generate .c files.
 #   6) compile_gperf, compile_re2c, etc. -- Actions that invoke programs that
 #              turn intermediate files into .c files.
 
 if (current_toolchain == host_toolchain) {
   # Various files referenced by multiple targets.
   yasm_gen_include_dir = "$target_gen_dir/include"
-  yasm_os = os
-  if (is_chromeos) {
-    yasm_os = "linux"
-  }
-  config_makefile = "source/config/$yasm_os/Makefile"
+  config_makefile = "source/config/$host_os/Makefile"
   version_file = "version.mac"
 
   import("//build/compiled_action.gni")
 
   config("yasm_config") {
     include_dirs = [
-      "source/config/$yasm_os",
+      "source/config/$host_os",
       "source/patched-yasm",
     ]
     defines = [ "HAVE_CONFIG_H" ]
     if (is_posix) {
       cflags = [ "-std=gnu99" ]
     }
   }
 
   executable("genmacro") {
     sources = [
@@ skipping 86 matching lines @@
 
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [
       ":yasm_config",
       "//build/config/compiler:no_chromium_code",
     ]
 
     # re2c is missing CLOSEVOP from one switch.
     if (is_posix) {
       cflags = [ "-Wno-switch" ]
-    } else if (is_win) {
-      cflags = [ "/wd4267" ]  # size_t to int conversion.
     }
   }
 
   executable("yasm") {
     sources = [
       "source/patched-yasm/frontends/yasm/yasm-options.c",
       "source/patched-yasm/frontends/yasm/yasm.c",
       "source/patched-yasm/libyasm/assocdat.c",
       "source/patched-yasm/libyasm/bc-align.c",
       "source/patched-yasm/libyasm/bc-data.c",
@@ skipping 78 matching lines @@
       ":yasm_config",
       "//build/config/compiler:no_chromium_code",
     ]
 
     # Yasm generates a bunch of .c files which its source file #include.
     # Add the |target_gen_dir| into the include path so it can find them.
     # Ideally, these generated .c files would be placed into a separate
     # 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",
       ]
       if (is_clang) {
         cflags += [ "-Wno-incompatible-pointer-types" ]
       }
     }
 
     # TODO(ajwong): This should take most of the generated output as
@@ skipping 207 matching lines @@
 
     # Output eventually #included by source/patched-yasm/frontends/yasm/x86id.c
     outputs = [
       "$yasm_gen_include_dir/x86insns.c",
       "$yasm_gen_include_dir/x86insn_gas.gperf",
       "$yasm_gen_include_dir/x86insn_nasm.gperf",
     ]
     args = [ rebase_path(yasm_gen_include_dir, root_build_dir) ]
   }
 }