String::FlatContent and FlatStringReader are ways of reading a flat
String. Both of them provide a speedy interface to read a String since
they bypass the dynamic dispatch of String::Get. For this very same
reason, they are not safe to be used from background threads.

Bug: v8:7790
Change-Id: I919ae5e3862df81ff3ebb5131144ff5961197b35
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2509596
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70945}