When most elements of an object are deleted, we want to normalize its
elements backing store to a dictionary in order to save space. Finding
the right time to do so should not incur a linear cost on each delete
operation. This patch changes the heuristic to an amortized-constant
approach based on a global counter and the current backing store
capacity.

BUG=chromium:542978

Change-Id: Ifdf29ab2211fdde1df9078f63be4118627d6a67e
Reviewed-on: https://chromium-review.googlesource.com/506191Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45330}