GN: Move variables from //build_overrides/webrtc.gni to //webrtc/build/webrtc.gni

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
-
-declare_args() {
-  # Include tests in standalone checkout.
-  rtc_include_tests = true
-}