It's unnecessary to buffer in the zone, and using a deque is more expensive
than an std::vector as a stack since we reuse areas very frequently. The
top-of-stack that the State keeps track of is now simply tracked in the state,
with a scope_depth_ counter to figure out if the fni_ is "open" (has an active
state).

Change-Id: I29ad3db7520340b8fe035feed400178bd50785bc
Reviewed-on: https://chromium-review.googlesource.com/c/1298894Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56974}