Permits CHECKing/DCHECKing that methods are being accessed in a
thread-safe manner, even if they are not used by one single thread
(thread pools such as VideoToolbox OK).
BUG=
R=danilchap@webrtc.org, tommi@webrtc.org
Review URL: https://codereview.webrtc.org/2097403002 .
Cr-Commit-Position: refs/heads/master@{#13358}
