For external uncached strings (also called "Small External Strings")
with cacheable resources, we can cache its resource's data at the
string's creation time. This allows us to safely read the data from the
background as we wouldn't trigger a data() callback.

For more information regarding the investigation and possible proposals
see
https://docs.google.com/document/d/101eAQqFpBPWFGNJicxtdlwYShJkTOUsEuxkVVeu5Hrk/edit?usp=sharing

Bug: v8:7790, v8:11463
Change-Id: I6164092b01a6ccb525a9516f476e066b35fb1f96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2685177
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72862}