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/rtp_rtcp/source/rtp_rtcp_impl.cc

@@ -40,6 +40,8 @@ RTPExtensionType StringToRtpExtensionType(const std::string& extension) {
     return kRtpExtensionTransportSequenceNumber;
   if (extension == RtpExtension::kPlayoutDelayUri)
     return kRtpExtensionPlayoutDelay;
+  if (extension == RtpExtension::kVideoContentTypeUri)
+    return kRtpExtensionVideoContentType;
   RTC_NOTREACHED() << "Looking up unsupported RTP extension.";
   return kRtpExtensionNone;
 }