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

build/config/compiler/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/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 
@@ skipping 796 matching lines @@
         exec_script("../../win_is_xtree_patched.py", [], "value") == 0) {
       cflags += [ "/wd4702" ]  # Unreachable code.
     }
 
     # Building with Clang on Windows is a work in progress and very
     # experimental. See crbug.com/82385.
     # Keep this in sync with the similar block in build/common.gypi
     if (is_clang) {
       cflags += [
         # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
+        "-Qunused-arguments",  # http://crbug.com/504658
         "-Wno-microsoft-enum-value",  # http://crbug.com/505296
         "-Wno-unknown-pragmas",  # http://crbug.com/505314
         "-Wno-microsoft-cast",  # http://crbug.com/550065
       ]
     }
   } else {
     if (is_mac && !is_nacl) {
       # When compiling Objective-C, warns if a method is used whose
       # availability is newer than the deployment target. This is not
       # required when compiling Chrome for iOS.
@@ skipping 392 matching lines @@
     #   }
     # So this config has to have the selection logic just like
     # "default_optimization", below.
     configs = [ "//build/config/nacl:irt_optimize" ]
   } else {
     ldflags = common_optimize_on_ldflags
     if (is_win) {
       # Favor speed over size, /O2 must be before the common flags. The GYP
       # build also specifies /Ot, /Oi, and /GF, but these are implied by /O2.
       cflags = [ "/O2" ] + common_optimize_on_cflags
-
-      # TODO(thakis): Remove is_clang here, https://crbug.com/598772
-      if (is_official_build && !is_clang) {
+      if (is_official_build) {
         # TODO(GYP): TODO(dpranke): Should these only be on in an official
         # build, or on all the time? For now we'll require official build so
         # that the compile is clean.
         cflags += [
           "/GL",  # Whole program optimization.
 
           # Disable Warning 4702 ("Unreachable code") for the WPO/PGO builds.
           # Probably anything that this would catch that wouldn't be caught in a
           # normal build isn't going to actually be a bug, so the incremental
           # value of C4702 for PGO builds is likely very small.
@@ skipping 104 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }