Reland of Add content type information to encoded images and corresponding rtp extension header (patchset #1 id:1 of https://codereview.webrtc.org/2809653004/ )

Reason for revert:
Fix failing bots.

BUG=webrtc:7420

Review-Url: https://codereview.webrtc.org/2816493002
Cr-Commit-Position: refs/heads/master@{#17658}
diff --git a/webrtc/modules/video_coding/frame_buffer.cc b/webrtc/modules/video_coding/frame_buffer.cc
index 1439a17..5ea12dc 100644
--- a/webrtc/modules/video_coding/frame_buffer.cc
+++ b/webrtc/modules/video_coding/frame_buffer.cc
@@ -163,6 +163,7 @@
     RTC_DCHECK(!_rotation_set);
     rotation_ = packet.video_header.rotation;
     _rotation_set = true;
+    content_type_ = packet.video_header.content_type;
   }
 
   if (packet.is_first_packet_in_frame) {