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/encoded_frame.h

@@ -77,8 +77,12 @@ class VCMEncodedFrame : protected EncodedImage {
   */
   VideoRotation rotation() const { return rotation_; }
   /**
-  *   True if this frame is complete, false otherwise
-  */
+   *  Get video content type
+   */
+  VideoContentType contentType() const { return content_type_; }
+  /**
+   *   True if this frame is complete, false otherwise
+   */
   bool Complete() const { return _completeFrame; }
   /**
   *   True if there's a frame missing before this frame