Created a mock AudioTransport.

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 a0f6746c538d386160dc8a0775448157f76083e0..c9c524a82dbc59b0bc7da0eb8dfd66c2e245a803 100644
--- a/webrtc/modules/audio_device/BUILD.gn
+++ b/webrtc/modules/audio_device/BUILD.gn
@@ -241,6 +241,28 @@ rtc_static_library("audio_device") {
   }
 }
 
+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",
+      ]
+    }
+  }
+}
+
 # These tests do not work on ios, see
 # https://bugs.chromium.org/p/webrtc/issues/detail?id=4755
 if (rtc_include_tests && !is_ios) {