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_object.cc b/webrtc/modules/video_coding/frame_object.cc
index 70b0a02..9e5ce09 100644
--- a/webrtc/modules/video_coding/frame_object.cc
+++ b/webrtc/modules/video_coding/frame_object.cc
@@ -79,6 +79,7 @@
   // (HEVC)).
   rotation_ = last_packet->video_header.rotation;
   _rotation_set = true;
+  content_type_ = last_packet->video_header.content_type;
 }
 
 RtpFrameObject::~RtpFrameObject() {