Make rtc_include_tests true by default in gn.

build_overrides/webrtc.gni

 # Copyright (c) 2015 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.
 
 # This file contains overrides to GN variables that applies for the WebRTC
 # standalone build (i.e. not in Chromium).
 #
 # Every variable here needs to be present in the corresponding file in
 # build_overrides/ of Chromium.
 
 # The build_with_chromium variable is now located in build.gni (now also used
 # by BoringSSL).
 import("//build_overrides/build.gni")
 
 # Excluded in Chromium since its prerequisites don't require Pulse Audio.
 rtc_include_pulse_audio = true
 
 # Chromium uses its own IO handling, so the internal ADM is only built for
 # standalone WebRTC.
 rtc_include_internal_audio_device = true
 
-# There's no point processing WebRTC standalone tests in a Chromium build.
-rtc_include_tests = true
+declare_args() {
+  # Include tests in standalone checkout.
+  rtc_include_tests = true
+}