Add passkey to TransformableFrameInterface to prevent external impls

This makes the downcasts currently used in eg
modules/rtp_rtcp/source/rtp_sender_video_frame_transformer_delegate.cc
safer.

Bug: webrtc:339815768
Change-Id: Ie6c7ab84666d399dbca4c95846b850aac5327f1a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/350361
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tony Herre <herre@google.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42325}

api/BUILD.gn

diff --git a/api/BUILD.gn b/api/BUILD.gn
index 464e1be..64028ad 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -393,7 +393,10 @@
 
 rtc_source_set("frame_transformer_interface") {
   visibility = [ "*" ]
-  sources = [ "frame_transformer_interface.h" ]
+  sources = [
+    "frame_transformer_interface.cc",
+    "frame_transformer_interface.h",
+  ]
   deps = [
     ":make_ref_counted",
     ":ref_count",