The new helper function allows us to write tests for log parsing
without the need to first generating a log file. This makes it easier
to spot errors when the log format changes.

- Add d8 global variable
- Add file_name accessor to Logger and Log classes
- Change OS::LogFileOpenMode to w+ / wb+
- Use separate Log::WriteLogHeader method
- Remove unused logger_ instance variable from Log

Bug: v8:10644
Change-Id: Ifc7e35aa4e91b3f01f0847843263946e085944c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2387563
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69715}