For testing log file parsing from JavaScript we need to be able to
read open log files directly. This makes the default log file mode
consistent with the temporary one.

Bug: v8:1064
Change-Id: Ic77ba6864efe633e54792fc683edc2fe07af4cd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2400993Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69783}