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}

webrtc/modules/video_coding/generic_decoder.h

@@ -30,6 +30,7 @@ struct VCMFrameInformation {
   int64_t decodeStartTimeMs;
   void* userData;
   VideoRotation rotation;
+  VideoContentType content_type;
 };
 
 class VCMDecodedFrameCallback : public DecodedImageCallback {
@@ -109,6 +110,7 @@ class VCMGenericDecoder {
   VideoCodecType _codecType;
   bool _isExternal;
   bool _keyFrameDecoded;
+  VideoContentType _last_keyframe_content_type;
 };
 
 }  // namespace webrtc