The backing store is only held alive indirectly via the array buffer
referenced by the holder (typed array), so it's not enough to keep the
elements alive (or even just the external pointer loaded from the
elements).

R=mstarzinger@chromium.org,bmeurer@chromium.org
LOG=n
BUG=v8:1827

Review URL: https://codereview.chromium.org/1493983004

Cr-Commit-Position: refs/heads/master@{#32644}