Now that the trace json file has changed name, update the extension
checked by the --retain=json flag in generate-runtime-callstats.py.

Bug: v8:10348
Change-Id: Ieb14b77d2d399a1246049170f289b4666658f376
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2122015
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66870}