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}
diff --git a/api/DEPS b/api/DEPS
index ef9db30..1e92b12 100644
--- a/api/DEPS
+++ b/api/DEPS
@@ -90,6 +90,10 @@
     "+modules/include/module_fec_types.h",
   ],
 
+  "frame_transformer_interface\.h": [
+    "+rtc_base/ref_count.h",
+  ],
+
   "ice_transport_interface\.h": [
     "+rtc_base/ref_count.h",
   ],