Each time a thread accesses a DEFINE_THREAD_SAFE_STATIC_LOCAL, it will leak an object:
https://cs.chromium.org/chromium/src/third_party/blink/renderer/platform/wtf/std_lib_extras.h?type=cs&q=DEFINE_THREAD_SAFE_STATIC_LOCAL&sq=package:chromium&g=0&l=46

If a process creates an destroys threads dynamically [and the threads access the variable], this will cause an unbounded memory leak over time.

I suspect this will mostly affect the task scheduler [if it ever ends up making/destroy thread dynamically].

We should get rid of the macro. It's possible to accomplish the same behavior without leaking in a few lines of code:

e.g.
"""
base::ThreadLocalStorage::Slot& ShimStateTLS() {
  static base::NoDestructor<base::ThreadLocalStorage::Slot> shim_state_tls(
      &DestructShimState);
  return *shim_state_tls;
}

ShimState* GetShimState() {
  ShimState* state = static_cast<ShimState*>(ShimStateTLS().Get());

  if (!state) {
    state = new ShimState();
    ShimStateTLS().Set(state);
  }

  return state;
}
"""