SeededNumberDictionaries are used to implement element backing stores of JSObjects, not internally used dictionaries. This saves space for the anyway unused PropertyDetails entry (1/3 fields).

Bug: 
Change-Id: I6fe9fae6de500dd0bcb722f51a7543952c7813e9
Reviewed-on: https://chromium-review.googlesource.com/543343
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46124}