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With this, you can make the compiler ensure, statically, that
the methods of your class always run on the same sequence.
This is vastly safer than runtime checks like SequenceChecker
since it will fail to compile if you violate the requirement.

(It wouldn't be C++ if there weren't hacky ways around this, but
any attempt to trick the compiler into allowing a sequence
violation should look pretty obvious to reviewers.)

To use this, declare a struct that inherits from StaticSequence.
This struct will automatically declare a Key class that can only
be constructed inside the StaticSequence's PostTask function. To
force users to go through that StaticSequence's PostTask() to
call your method, simply declare a reference to the Key object
as the last parameter to the method.

This also includes a wrapper template called Sequenced, similar
to base::SequenceBound, but with the TaskRunner known at compile
time rather than runtime. This can be used to add an extra level
of safety over runtime checks like
DCHECK_CALLED_ON_VALID_SEQUENCE or
TaskRunner::RunsTasksInCurrentThread. This wrapper ensures the
wrapped object is destroyed on the correct sequence, essentially
turning any sequence-affine object to a thread-safe object.

This will be made even more useful once base::PostTask() returns
a base::Promise, as it will work with methods that return values
as well as void methods.

Bug: None
Test: cast_base_unittests
Change-Id: Ic408e343bc084c19c4c6f9e983b343b98502daf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863549Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711007}