Create public EncodedFrame interface.

The plan is to:
 1. Move FrameObject to api/video.
 2. Rename FrameObject to EncodedFrame.
 3. Move EncodedFrame out of the video_coding namespace.

This is the 1st CL.

Bug: webrtc:8909
Change-Id: I2e5100eda6c51bcefb32295e03b73cf1f5c213a4
Reviewed-on: https://webrtc-review.googlesource.com/55560
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22135}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 8d34b58..280b7e7 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -238,6 +238,17 @@
   ]
 }
 
+rtc_source_set("encoded_frame_api") {
+  visibility = [ "*" ]
+  sources = [
+    "video/encoded_frame.h",
+  ]
+
+  deps = [
+    "../modules/video_coding:encoded_frame",
+  ]
+}
+
 rtc_source_set("video_frame_api_i420") {
   visibility = [ "*" ]
   sources = [