Remove the 'ios_use_webrtc' build flag hack.

build_overrides/webrtc.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 build_with_chromium = true
 
 # Exclude pulse audio on Chromium since its prerequisites don't require
 # pulse audio.
 rtc_include_pulse_audio = false
 
 # Exclude internal ADM since Chromium uses its own IO handling.
 rtc_include_internal_audio_device = false
-
-declare_args() {
-  # TODO(dpranke): This is a hack needed to get iOS to build w/ a
-  # patched version of WebRTC so that we can access the xmllite
-  # library. Remove this once we've figured out how to get WebRTC building
-  # properly.
-  ios_use_webrtc = false
-}