lacros: always log to the same file.

Previously, the log was dumped to a file with the current timestamp.
This proved unwieldy. Now we always log to the same file, and delete it
if lacros is launching for the first time.

Change-Id: I1a5d321abf14e62e1cc8464b0d3a36647f0cdd06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227438
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775784}