This CL makes X11 optional for audio_device_pulse_linux. Apply the same guards than for audio_device_alsa_linux. Cf commit 75f18fca8eef7c27073923c46ff73be5ba0e0491. Bug: webrtc:9569 Change-Id: Iaddbfb62112c504531376bad0db8f04caa4350c7 Reviewed-on: https://webrtc-review.googlesource.com/93030 Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org> Reviewed-by: Patrik Höglund <phoglund@webrtc.org> Commit-Queue: Yves Gerey <yvesg@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24274}