Add content type information to Encoded Images and add corresponding RTP extension header.

Use it to separate UMA e2e delay metric between screenshare from video.
Content type extension is set based on encoder settings and processed and decoders.

Also,
Fix full-stack-tests to calculate RTT correctly, so new metric could be tested.

BUG=webrtc:7420

Review-Url: https://codereview.webrtc.org/2772033002
Cr-Commit-Position: refs/heads/master@{#17640}

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