We currently grow the backing store to (old_capacity*1.5)+16 if we exceed capacity, 
but shrink the capacity to the current length when 2*length <= capacity.
For short arrays (up to length 32), this can lead to a copy on every operation when using push/pop or push/shift.

Example:
Array of length 32, capacity 32
push
Array grown to length 33, capacity 32*1.5+16 = 64
pop
Array trimmed to length 32, capacity 32 because 2*32 <= 64
...

This CL leaves additional slag space when calling pop and restricts the trimming to backing stores with at least 16 elements to prevent excessive re-trimming on short arrays.

Bug: 
Change-Id: I9dd13e5e2550c7ac819294c8e29f04c8855e02a4
Reviewed-on: https://chromium-review.googlesource.com/502911
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45324}