Roll buildtools forward and fix GN build for it.

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 454cddea4f53249c775e17e62900a0f0a990e107..d71098f623a37daff46899db39d755bacf8ccd33 100644
--- a/build/toolchain/win/BUILD.gn
+++ b/build/toolchain/win/BUILD.gn
@@ -121,7 +121,7 @@ template("msvc_toolchain") {
     tool("asm") {
       # 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 $env ml.exe {{defines}} {{include_dirs}} /safeseh /c /Fo {{output}} {{source}}"
+      command = "$python_path gyp-win-tool asm-wrapper $env ml.exe {{defines}} {{include_dirs}} {{asmflags}} /safeseh /c /Fo {{output}} {{source}}"
       description = "ASM {{output}}"
       outputs = [
         "{{target_out_dir}}/{{target_output_name}}/{{source_name_part}}.obj",