This CL introduces TaskToken to identify individual tasks. In the scope
of a ScopedSetSequenceTokenForCurrentThread, a unique TaskToken is set
in TLS. This unique TaskToken can be retrieved using
TaskToken::GetForCurrentThread().

ThreadCheckerImpl uses TaskToken to make CalledOnValidThread() return
true when called multiple times from the same task. This allows usage
of ThreadChecker/NonThreadSafe objects from the stack on tasks not
otherwise running in a single-threaded context.

BUG=553459

Review-Url: https://codereview.chromium.org/2213263002
Cr-Commit-Position: refs/heads/master@{#410839}