This allows (among other things) type-checked down-casts (similar to dynamic_cast) This will be used in a follow-up CL. This CL also moves some one-liner functions from the .cc file to the .h file. Bug: webrtc:11933 Change-Id: Ic89de8fa6c445ecbe108f2fbf68b44b655f819f5 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/199970 Reviewed-by: Artem Titov <titovartem@webrtc.org> Commit-Queue: Björn Terelius <terelius@webrtc.org> Cr-Commit-Position: refs/heads/master@{#32905}