GN: Fix Windows Clang errors

webrtc/test/BUILD.gn

 # Copyright (c) 2014 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/ui.gni")
 import("//testing/test.gni")
@@ skipping 399 matching lines @@
     "//testing/gtest",
     "//third_party/gflags",
   ]
 }
 
 config("test_renderer_exported_config") {
   if (is_win && is_clang) {
     # 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 cannot be on the target directly.
-    cflags += [
+    cflags = [
       "-Wno-bool-conversion",
       "-Wno-comment",
       "-Wno-delete-non-virtual-dtor",
     ]
   }
 }
 
 source_set("test_renderer") {
   testonly = true
   libs = []
@@ skipping 51 matching lines @@
     configs -= [ "//build/config/clang:find_bad_constructs" ]
   }
 
   deps = [
     ":test_support",
     ":video_test_common",
     "../modules/media_file",
     "//testing/gtest",
   ]
 }