Injects FrameEncryptorInterface into RtpSender.

This change injects the FrameEncryptorInterface and the FrameDecryptorInterface
into the RtpSenderInterface and RtpReceiverInterface respectively. This is the
second stage of the injection. In a follow up CL non owning pointers to these
values will be passed down into the media channel.

This change also updates the corresponding mock files.

Bug: webrtc:9681
Change-Id: I964084fc270e10af9d1127979e713493e6fbba7d
Reviewed-on: https://webrtc-review.googlesource.com/96625
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24489}
diff --git a/api/rtpsenderinterface.cc b/api/rtpsenderinterface.cc
new file mode 100644
index 0000000..bbf3901
--- /dev/null
+++ b/api/rtpsenderinterface.cc
@@ -0,0 +1,23 @@
+/*
+ *  Copyright 2018 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.
+ */
+
+#include "api/rtpsenderinterface.h"
+
+namespace webrtc {
+
+void RtpSenderInterface::SetFrameEncryptor(
+    rtc::scoped_refptr<FrameEncryptorInterface> frame_encryptor) {}
+
+rtc::scoped_refptr<FrameEncryptorInterface>
+RtpSenderInterface::GetFrameEncryptor() const {
+  return nullptr;
+}
+
+}  // namespace webrtc