Revert of clang/win: Stop passing -Qunused-arguments.

build/config/win/BUILD.gn

Index: build/config/win/BUILD.gn
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index d2792ef320df90e001f2b49b178dd2f20d17c460..7945cfc10066c4a1d15bdc649d37ab67b84ea6d7 100644
--- a/build/config/win/BUILD.gn
+++ b/build/config/win/BUILD.gn
@@ -33,13 +33,10 @@
 
   cflags = [
     "/Gy",  # Enable function-level linking.
+    "/GS",  # Enable buffer security checking.
     "/FS",  # Preserve previous PDB behavior.
     "/bigobj",  # Some of our files are bigger than the regular limits.
   ]
-  if (!is_clang) {
-    # TODO(thakis): Enable this with clang too, https://crbug.com/598767
-    cflags += [ "/GS" ]  # Enable buffer security checking.
-  }
 
   # Force C/C++ mode for the given GN detected file type. This is necessary
   # for precompiled headers where the same source file is compiled in both