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.

Bug: chromium:771227
Change-Id: I8aa1514b37a74f82539f95f94292c8fa1582d66a
Reviewed-on: https://chromium-review.googlesource.com/789511Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49693}