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}