Add FrameEncryptor/FrameDecryptor support to Objective C API for WebRTC.

This change adds bindings so that native FrameEncryptor and native FrameDecryptor
objects can be set on the objective C RTCRtpSender and RTCRtpReceiver objects.

Bug: webrtc:9681
Change-Id: Iec4006ea020d6ab6adcc0ad068dcd8fb2738063d
Reviewed-on: https://webrtc-review.googlesource.com/c/103020
Commit-Queue: Benjamin Wright <benwright@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24936}
diff --git a/sdk/objc/api/peerconnection/RTCRtpReceiver+Native.h b/sdk/objc/api/peerconnection/RTCRtpReceiver+Native.h
new file mode 100644
index 0000000..33ba40b
--- /dev/null
+++ b/sdk/objc/api/peerconnection/RTCRtpReceiver+Native.h
@@ -0,0 +1,30 @@
+/*
+ *  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.
+ */
+
+#import "RTCRtpReceiver.h"
+
+#include "api/crypto/framedecryptorinterface.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ * This class extension exposes methods that work directly with injectable C++ components.
+ */
+@interface RTCRtpReceiver ()
+
+/** Sets a user defined frame decryptor that will decrypt the entire frame.
+ * This will decrypt the entire frame using the user provided decryption
+ * mechanism regardless of whether SRTP is enabled or not.
+ */
+- (void)setFrameDecryptor:(rtc::scoped_refptr<webrtc::FrameDecryptorInterface>)frameDecryptor;
+
+@end
+
+NS_ASSUME_NONNULL_END