This CL adds an Allocator to SmallVector to control how dynamic
storage is managed. The default value uses the plain old C++
std::allocator<T>, i.e. acts like malloc/free.

For use with zone memory, one can pass a ZoneAllocator as follows:

  // Allocates in zone memory.
  base::SmallVector<int, kInitialSize, ZoneAllocator<int>>
    xs(ZoneAllocator<int>(zone));

Note: this is a follow-up to crrev.com/c/3240823.

Drive-by: hide the internal `reset` function. It doesn't free the
dynamic backing store; that's a surprise and should not be exposed to
external use.

Change-Id: I1f92f184924541e2269493fb52c30f2fdec032be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3257711
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77755}