We currently never shrink the StringTable which causes excessive memory usage
on certain websites. This CL tries to mitigate this by shrinking the
StringTable if it is very empty (nof_elements * 16 < capacity) hopefully
avoiding costly reallocations.

Bug: chromium:818642, v8:5443
Change-Id: I4e6a95b3a6992b499fa6dd59ae159c51f089965a
Reviewed-on: https://chromium-review.googlesource.com/970465Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52070}