GN: Add audio_conference_mixer_unittests to modules_unittests.

webrtc/modules/audio_conference_mixer/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/webrtc.gni")
 
@@ skipping 40 matching lines @@
   rtc_source_set("audio_conference_mixer_unittests") {
     testonly = true
     sources = [
       "test/audio_conference_mixer_unittest.cc",
     ]
     deps = [
       ":audio_conference_mixer",
       "../../test:test_support",
       "//testing/gmock",
     ]
+    if (is_win) {
+      cflags = [
+        # TODO(kjellander): bugs.webrtc.org/261: Fix this warning.
+        "/wd4373",  # virtual function override.
+      ]
+    }
     if (!build_with_chromium && is_clang) {
       # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
       suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
     }
   }
 }