win gn: Use /safeseh, not /SAFESEH

[14421->5901/20355 ~33] ASM obj/third_party/crashpad/crashpad/util/util/capture_context.obj
MASM : warning A4018:invalid command-line option : /SAFESEH
MASM : warning A4018:invalid command-line option : /SFESEH
MASM : warning A4018:invalid command-line option : /SESEH
MASM : warning A4018:invalid command-line option : /SSEH
MASM : warning A4018:invalid command-line option : /SEH
MASM : warning A4018:invalid command-line option : /SH

BUG=546288
R=brettw@chromium.org

Review URL: https://codereview.chromium.org/1470483002

Cr-Original-Commit-Position: refs/heads/master@{#360960}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 7ba9b478144998dc08029cddf61e8a48745fef9a

config/win/BUILD.gn

@@ -13,11 +13,12 @@ assert(is_win)
 # Windows-only.
 config("compiler") {
   asmflags = [
-    # When /SAFESEH is specified, the linker will only produce an image if it
+    # When /safeseh is specified, the linker will only produce an image if it
     # can also produce a table of the image's safe exception handlers. This
     # table specifies for the operating system which exception handlers are
-    # valid for the image.
-    "/SAFESEH",
+    # valid for the image. Note that /SAFESEH isn't accepted on the command
+    # line, only /safeseh.
+    "/safeseh",
   ]
 
   cflags = [