Revert of [GN]: BUILD file housecleaning

build/toolchain/win/BUILD.gn

Index: build/toolchain/win/BUILD.gn
diff --git a/build/toolchain/win/BUILD.gn b/build/toolchain/win/BUILD.gn
index 7a0ecbb4ad89e9e28e4c8a1797199aaf6ffe721e..a0542678ee6806c1df951e9e33fef9071e24ac4e 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -120,7 +120,9 @@ template("msvc_toolchain") {
     }
 
     tool("asm") {
-      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} {{asmflags}} /c /Fo{{output}} {{source}}"
+      # TODO(brettw): "/safeseh" assembler argument is hardcoded here. Extract
+      # assembler flags to a variable like cflags. crbug.com/418613
+      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} /safeseh /c /Fo {{output}} {{source}}"
       description = "ASM {{output}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.obj",