gn: Fix more build issues on Win

build/toolchain/win/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/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 
 # Should only be running on Windows.
 assert(is_win)
 
@@ skipping 53 matching lines @@
 
     tool("rc") {
       command = "$python_path gyp-win-tool rc-wrapper $env rc.exe {{defines}} {{include_dirs}} /fo{{output}} {{source}}"
       outputs = [
         "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.res",
       ]
       description = "RC {{output}}"
     }
 
     tool("asm") {
-      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} /c /Fo {{output}} {{source}}"
+      # TODO(brettw): "/safeseh" assembler argument is hardcoded here. Extract
+      # assembler flags to a variable like cflags. crbug.com/418613
+      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} /safeseh /c /Fo {{output}} {{source}}"
       description = "ASM {{output}}"
       outputs = [
         "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
       ]
     }
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
       command = "$python_path gyp-win-tool link-wrapper $env False lib.exe /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
@@ skipping 80 matching lines @@
 msvc_toolchain("32") {
   environment = "environment.x86"
   cpu_arch = "x64"
 }
 
 msvc_toolchain("64") {
   environment = "environment.x64"
   cpu_arch = "x64"
   force_win64 = true
 }