{do_raw_json} and {do_json} both read the log files to construct a dictionary of
stats.

This CL extracts that logic and eliminates code duplication

No-Try: true
Bug: v8:9448
Change-Id: I375920c25942a92cc12790ac60a4c7960cfd44b2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1706473
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62768}