Since the StringTable can only contain old strings, skip iterating it
when SkipRoot::kOldGeneration is set.

Bug: chromium:1115132, chromium:1115100
Change-Id: I6d065a0ea7f3142c5d474eb0919e801e13976f6c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2351664
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69353}