1. Use `StringBuffer` instead of `StringView` in `WebDriverValue`.
2. Add some `DCHECK`s.
3. Reserve vector size.
4. Respect properties with `undefined` values.
5. Minor clean-ups.

Change-Id: Ic109acb1e3adf2d950767173c17a9203e3c816dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3596173Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Auto-Submit: Maksim Sadym <sadym@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80296}