CL in preparation of writing JavaScript-based log parsing tests.

- Return both temporary and normal log file in
  Log::TearDownAndGetLogFile
- Add file_name accessor to Logger and Log classes
- Use separate Log::WriteLogHeader method
- Remove unused logger_ instance variable from Log

Bug: v8:10668
Change-Id: Ie1f6f92cc6c55fd1dc664cac95f481bc29da7e18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2407773Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69879}