Cast resource field in ExternalString as

v8: :String::ExternalStringResourceBase* would give us more info.
Change-Id: Iae97b477f400f58365e2381b7230d2226d490aa7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388742
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#69734}