Protect APM in webkit builds.

Update libwertc AudioRtpSender::SetAudioSend with WEBRTC_WEBKIT_BUILD

This only introduces the WEBRTC_WEBKIT BUILD, inspired by WEBRTC_CHROMIUM_BUILD
macro. It is only defined by Webkit libwebrtc build system.
https://trac.webkit.org/changeset/210977

BUG=webrtc:7039

Review-Url: https://codereview.webrtc.org/2651273003
Cr-Commit-Position: refs/heads/master@{#16432}
diff --git a/AUTHORS b/AUTHORS
index e9437d0..1c23c3e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -3,6 +3,7 @@
 
 Adam Fedor <adam.fedor@gmail.com>
 Alexander Brauckmann <a.brauckmann@gmail.com>
+Alexandre Gouaillard <agouaillard@gmail.com>
 Andrew MacDonald <andrew@webrtc.org>
 Anil Kumar <an1kumar@gmail.com>
 Ben Strong <bstrong@gmail.com>