Reland of Add content type information to encoded images and corresponding rtp extension header

webrtc/modules/video_coding/encoded_frame.h

Index: webrtc/modules/video_coding/encoded_frame.h
diff --git a/webrtc/modules/video_coding/encoded_frame.h b/webrtc/modules/video_coding/encoded_frame.h
index 840cd2056d594562799957559948f461e24b56a7..96f9d00d06e77780e9e6b5ac7b9f3125d63396f1 100644
--- a/webrtc/modules/video_coding/encoded_frame.h
+++ b/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