- Make it possible to store quarter-bytes instead of full bytes.

- Don't store is_used; it can be recovered correctly based on the actual full
  parse (when a lazy function is eventually called) and
  has_forced_scope_allocation.

- With the is_used change, the old testing approach (which compared a scope for
  which we didn't do scope allocation to the baseline) no longer made
  sense. Replaced it with a new testing approach, which is also closer to the
  actual usage.

- First version (reverted): https://chromium-review.googlesource.com/725422

BUG=v8:5516

Change-Id: I1468af6670b689a104bd867377caa1d236070820
Reviewed-on: https://chromium-review.googlesource.com/733123Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48903}