Before https://crrev.com/c/3829541, ReduceStringPrototypeStartsWith
would not be called if the String's content wasn't safe to access in
the background, because StringRef::length would fail in that case. Now
that StringRef::length always succeeds, an additional check is
required before calling ReduceStringPrototypeStartsWith.

Note that none of the other callers of StringRef::length access the
String's content later, so we shouldn't have any more bugs caused by
the aforementioned CL.

Bug: chromium:1354439
Change-Id: I4a590ccdb7cc4c8a85e4e6beaf6f3c3ab2d7d479
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3840938
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82592}