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}
diff --git a/webrtc/modules/video_coding/encoded_frame.h b/webrtc/modules/video_coding/encoded_frame.h
index 840cd20..96f9d00 100644
--- a/webrtc/modules/video_coding/encoded_frame.h
+++ b/webrtc/modules/video_coding/encoded_frame.h
@@ -77,8 +77,12 @@
   */
   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