Introduce frame transformer interfaces for Insertable Streams Web API.

Define FrameTransformerInterface for transforming encoded frames, and
TransformedFrameCallback for receiving transformed frames.

The FrameTransformerInterface will be implemented on the browser side,
and will be set in WebRTC sender and receiver in follow up CLs:
- Sender: https://webrtc-review.googlesource.com/c/src/+/169127
- Receiver: https://webrtc-review.googlesource.com/c/src/+/169129/1

Insertable Streams Web API explainer:
https://github.com/alvestrand/webrtc-media-streams/blob/master/explainer.md

Design doc for WebRTC library changes:
http://doc/1eiLkjNUkRy2FssCPLUp6eH08BZuXXoHfbbBP1ZN7EVk

Bug: webrtc:11380
Change-Id: Icf8ff159feb604f006e18157660f13d300a08b2b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169126
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30637}

api/BUILD.gn

@@ -162,6 +162,7 @@ rtc_library("libjingle_peerconnection_api") {
     ":audio_options_api",
     ":callfactory_api",
     ":fec_controller_api",
+    ":frame_transformer_interface",
     ":libjingle_logging_api",
     ":media_stream_interface",
     ":network_state_predictor_api",
@@ -213,6 +214,16 @@ rtc_library("libjingle_peerconnection_api") {
   ]
 }
 
+rtc_source_set("frame_transformer_interface") {
+  visibility = [ "*" ]
+  sources = [ "frame_transformer_interface.h" ]
+  deps = [
+    ":scoped_refptr",
+    "../rtc_base:refcount",
+    "video:encoded_frame",
+  ]
+}
+
 rtc_library("rtc_error") {
   visibility = [ "*" ]
   sources = [

api/DEPS

@@ -90,6 +90,10 @@ specific_include_rules = {
     "+modules/include/module_fec_types.h",
   ],
 
+  "frame_transformer_interface\.h": [
+    "+rtc_base/ref_count.h",
+  ],
+
   "ice_transport_interface\.h": [
     "+rtc_base/ref_count.h",
   ],

api/frame_transformer_interface.h

@@ -0,0 +1,58 @@
+/*
+ *  Copyright 2020 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.
+ */
+
+#ifndef API_FRAME_TRANSFORMER_INTERFACE_H_
+#define API_FRAME_TRANSFORMER_INTERFACE_H_
+
+#include <memory>
+#include <vector>
+
+#include "api/scoped_refptr.h"
+#include "api/video/encoded_frame.h"
+#include "rtc_base/ref_count.h"
+
+namespace webrtc {
+
+// Objects implement this interface to be notified with the transformed frame.
+class TransformedFrameCallback : public rtc::RefCountInterface {
+ public:
+  virtual void OnTransformedFrame(
+      std::unique_ptr<video_coding::EncodedFrame> transformed_frame) = 0;
+
+ protected:
+  ~TransformedFrameCallback() override = default;
+};
+
+// Transformes encoded frames. The transformed frame is sent in a callback using
+// the TransformedFrameCallback interface (see below).
+class FrameTransformerInterface : public rtc::RefCountInterface {
+ public:
+  // Transforms |frame| using the implementing class' processing logic.
+  // |additional_data| holds data that is needed in the frame transformation
+  // logic, but is not included in |frame|; for example, when the transform
+  // function is used for encrypting/decrypting the frame, the additional data
+  // holds the serialized generic frame descriptor extension calculated in
+  // webrtc::RtpDescriptorAuthentication, needed in the encryption/decryption
+  // algorithms.
+  virtual void TransformFrame(std::unique_ptr<video_coding::EncodedFrame> frame,
+                              std::vector<uint8_t> additional_data,
+                              uint32_t ssrc) = 0;
+
+  virtual void RegisterTransformedFrameCallback(
+      rtc::scoped_refptr<TransformedFrameCallback>) = 0;
+  virtual void UnregisterTransformedFrameCallback() = 0;
+
+ protected:
+  ~FrameTransformerInterface() override = default;
+};
+
+}  // namespace webrtc
+
+#endif  // API_FRAME_TRANSFORMER_INTERFACE_H_