Remove build_overrides/webrtc.gni

It's no longer used and the same functionality can be accomplished
by adding variable defaults to the .gn default_args dict.

Bug: 768723
Change-Id: I5f6723c2831fb0a5c9eba24ba53b3152ed3c8352
Reviewed-on: https://chromium-review.googlesource.com/684175Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Henrik Kjellander <kjellander@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504336}

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.
-
-# This is needed for backwards-compatibility w/ the WebRTC repo for
-# the build_with_chromium flag.
-import("//build_overrides/build.gni")
-
-# 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
-
-# Don't include WebRTC tests.
-rtc_include_tests = false