Added a public GN config to compile mock headers.

webrtc/modules/audio_device/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 223 matching lines @@
       "dummy/file_audio_device_factory.h",
     ]
   }
 
   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" ]
   }
 }
 
+config("mock_audio_device_config") {
+  if (is_win) {
+    cflags = [
+      # TODO(phoglund): get rid of 4373 supression when
+      # http://code.google.com/p/webrtc/issues/detail?id=261 is solved.
+      # legacy warning for ignoring const / volatile in signatures.
+      "/wd4373",
+    ]
+  }
+}
+
 if (rtc_include_tests) {
   rtc_source_set("mock_audio_device") {
     testonly = true
     sources = [
       "include/mock_audio_device.h",
       "include/mock_audio_transport.h",
     ]
     deps = [
       ":audio_device",
       "../../test:test_support",
     ]
-    if (is_win) {
-      cflags = [
-        # TODO(phoglund): get rid of 4373 supression when
-        # http://code.google.com/p/webrtc/issues/detail?id=261 is solved.
-        # legacy warning for ignoring const / volatile in signatures.
-        "/wd4373",
-      ]
-    }
+    all_dependent_configs = [ ":mock_audio_device_config" ]
   }
 }
 
 # These tests do not work on ios, see
 # https://bugs.chromium.org/p/webrtc/issues/detail?id=4755
 if (rtc_include_tests && !is_ios) {
   rtc_executable("audio_device_tests") {
     testonly = true
     sources = [
       "test/audio_device_test_api.cc",
       "test/audio_device_test_defines.h",
     ]
     deps = [
       ":audio_device",
       "../..:webrtc_common",
       "../../system_wrappers",
       "../../test:test_support",
       "../../test:test_support_main",
       "../rtp_rtcp",
       "../utility",
       "//testing/gtest",
     ]
     public_configs = [ ":audio_device_config" ]
   }
 }