Once a buffer has been externalized, V8 is no longer responsible for managing
the memory. The fact that V8 was freeing was leading to double free errors once
Blink's GC got around to freeing the buffer too.

Bug: chromium:730171, chromium:731046
Change-Id: Ib18a7e37cafd51bce0c5a983d5cf8f3e64eb2c13
Reviewed-on: https://chromium-review.googlesource.com/530132
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45882}