This speeds up the json parser by 10-20%

Bug: chromium:771227
Change-Id: Ib2392471bdd9ff9041237708cb272229b5ece410
Reviewed-on: https://chromium-review.googlesource.com/700494
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48287}