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/encoded_frame.cc b/webrtc/modules/video_coding/encoded_frame.cc
index fb12c5b..1807fa5 100644
--- a/webrtc/modules/video_coding/encoded_frame.cc
+++ b/webrtc/modules/video_coding/encoded_frame.cc
@@ -87,6 +87,7 @@
   _codecSpecificInfo.codecType = kVideoCodecUnknown;
   _codec = kVideoCodecUnknown;
   rotation_ = kVideoRotation_0;
+  content_type_ = VideoContentType::UNSPECIFIED;
   _rotation_set = false;
 }