Added a public GN config to compile mock headers.

webrtc/modules/audio_device/BUILD.gn

Index: webrtc/modules/audio_device/BUILD.gn
diff --git a/webrtc/modules/audio_device/BUILD.gn b/webrtc/modules/audio_device/BUILD.gn
index c9c524a82dbc59b0bc7da0eb8dfd66c2e245a803..63def0545137e55fed377217bcac2ec4229c95e7 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -241,6 +241,17 @@ rtc_static_library("audio_device") {
   }
 }
 
+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
@@ -252,14 +263,7 @@ if (rtc_include_tests) {
       ":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" ]
   }
 }