Enforce WARN_UNUSED_RESULT attribute on Windows

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/toolchain/ccache.gni")
 
 if (current_cpu == "arm") {
@@ skipping 557 matching lines @@
 
     # 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-unused-value",  # http://crbug.com/505318
+
+        # Disable unused-value (crbug.com/505318) except -Wunused-result.
+        "-Wno-unused-value",
+        "-Wunused-result",
       ]
     }
   } else {
     # Common GCC warning setup.
     cflags += [
       # Enables.
       "-Wendif-labels",  # Weird old-style text after an #endif.
       "-Werror",  # Warnings as errors.
 
       # Disables.
@@ skipping 524 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }