Rename root kTop to kStackRoots

IMHO kStackRoots is more descriptive than kTop.

Change-Id: I9eeffa6974ae0188021cb1628c2b21e691ab9490
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2810782


Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73834}