- Check that we only store internalized strings for context-allocated
  function names
- Fix call to FunctionContextSlotIndex from V8HeapExplorer that could
  end up passing in a non-internalized string

Bug: chromium:1255105
Change-Id: Ie8bd5577bd0086241d47991fbe285f5d64ae3d4a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3245113Reviewed-by: Kim-Anh Tran <kimanh@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77540}