The parser holds a single vector whose backing storage is reused in calls
to ParseJsonObject, so that once we reach the peak number of unstored
properties no more allocations are required.

This improves performance of parsing inputs like those in Speedometer VanillaJS
by about 2% in my local measurement, and would presumably do better on more
pathological inputs.

This should also have the side effect of reducing peak memory usage at this time
slightly, since we do fewer zone allocations which cannot be freed until the
parse finishes.

Reland switches to use std::vector::data instead of operator[] to avoid an index
check in debug MSVC. In such cases the out-of-bounds pointer cannot be
dereferenced, so it is legal.

Bug: chromium:771227
Change-Id: I21837196372c904bfc799cd14353a73d11dcff32
Reviewed-on: https://chromium-review.googlesource.com/804062Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49997}