callstats.html creates grouped entries on the fly. Thus we can safely
ignore already added group entries from the input file.

Change-Id: I5a17fc895c4d36bfd7b79fcdb6d4644498998f86
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890977
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83135}