In {EmptyBackingStore}, the {free_on_destruct} flag was not set as an
optimization: Since there is no memory, it also does not have to be
freed. However, this flag has a side-effect: any backing store where
this flag is not set is considered {external}. The {external} flag is
mis-used by blink to indicate if ArrayBuffers need to be wrapped or not.

With this CL we set the {free_on_destruct} flag in {EmptyBackingStore},
but we change the ArrayBufferTracker to just ignore empty backing
stores.

R=ulan@chromium.org

Bug: chromium:1008840
Change-Id: I1552a6e013c8b23f39fba1c2d9d9c61dc30c0c74
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924263Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65067}