The *Checker helpers have DoNothing implementations for non-DCHECK builds, which are intended to have them incur zero overhead in production binaries. C++ isn't actually allowed to allocate zero bytes for members of empty-class types, though, so this doesn't quite have the desired effect.

fdoray@ proposed that we replace the classes & checks with macros, so they can be compiled out entirely, without depending on any compiler cleverness:

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1) Introduce THREAD/SEQUENCE_CHECKER and ASSERT_CALLED_ON_VALID_THREAD/SEQUENCE macros that can be used like this:

class A {
 public:
  void MyMethod() {
    ASSERT_CALLED_ON_VALID_THREAD(thread_checker_);
  }

 private:
  THREAD_CHECKER(thread_checker_);
};

These macros are expanded to nothing when !DCHECK_IS_ON().

2) Move existing uses of ThreadChecker and SequenceChecker to these macros.
3) For every ~230 instance of base::NonThreadSafe (https://cs.chromium.org/search/?q=base::NonThreadSafe&p=2&type=cs), upload a CL that replaces the base::NonThreadSafe inheritance with a SEQUENCE_CHECKER member. In the CL message, ask the reviewer to validate that the use of SEQUENCE_CHECKER instead of THREAD_CHECKER is appropriate (with documentation of differences between the two options). Use an automated script to switch to THREAD_CHECKER when the reviewer asks for it.
4) Remove base::NonThreadSafe.
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