This CL fixes a bug where we would accidentally shrink to the same size of
the StringTable causing repeated unecessary allocations.

Bug: v8:5443, chromium:818642
Change-Id: I353b179616d5293f6d7143e7381ae6711343a835
Reviewed-on: https://chromium-review.googlesource.com/1044207Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53012}