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This CL adds a type-safe set container for dense integers, following
std::set's API.

Currently, there are multiple use-cases in Autofill of the following
two patterns to express sets of small, dense integers:

Pattern 1:
  enum class E { A = 1, B = 2, C = 3, ... };
  std::set<E> set;
  set.insert(B);

Pattern 2:
  enum E { A = 1 << 0, B = 1 << 2, C = 1 << 3, ... };
  uint32_t bitmask = 0;
  bitmask |= B;

This CL's container combines the type-safety of the former pattern
with the efficiency of the latter.

Bug: 1007974
Change-Id: Ida9b9cec244119a27eca2c171b4fb2f52443d099
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350423
Commit-Queue: Christoph Schwering <schwering@google.com>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820983}