Whenever left-trimming is possible (e.g. whenever user code is
called), we must not store a reference to an exposed JSArray's
elements.

Bug: chromium:897366,v8:7382
Change-Id: I8dd6a93aa6ed19e755ccce7122e0e019dc578a31
Reviewed-on: https://chromium-review.googlesource.com/c/1292066Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56903}