GN: Fix Windows Clang errors

webrtc/media/BUILD.gn

 # Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/linux/pkg_config.gni")
 import("../build/webrtc.gni")
@@ skipping 261 matching lines @@
     public_deps += [ "//testing/gmock" ]
   }
 
   config("rtc_media_unittests_config") {
     # GN orders flags on a target before flags from configs. The default config
     # adds -Wall, and this flag have to be after -Wall -- so they need to
     # come from a config and can"t be on the target directly.
     # TODO(kjellander): Make the code compile without disabling these flags.
     # See https://bugs.webrtc.org/3307.
     if (is_clang && is_win) {
-      cflags += [ "-Wno-unused-function" ]
+      cflags = [ "-Wno-unused-function" ]
     }
     if (!is_win) {
       cflags = [ "-Wno-sign-compare" ]
       cflags_cc = [ "-Wno-overloaded-virtual" ]
     }
   }
 
   test("rtc_media_unittests") {
     testonly = true
 
@@ skipping 64 matching lines @@
     deps += [
       # TODO(kjellander): Move as part of work in bugs.webrtc.org/4243.
       ":rtc_media",
       ":rtc_unittest_main",
       "../audio",
       "../base:rtc_base_tests_utils",
       "../system_wrappers:metrics_default",
     ]
   }
 }