This simplifies mode processing as follows:
- Passing the --mode parameter is deprecated.
- The build output is now only searched in the --outdir parameter
that was passed (previously some combinations of mode and outdir
were possible).
- The mode is deduced from the build artifacts based on the gn
arguments "is_debug" and "dcheck_always_on".
- Timeouts and status file entries in release mode with dchecks are
treated like in debug mode.

This change was prepared on the infrastructure side by deprecating
the --mode flag and passing --outdir=out/build:
https://crrev.com/c/2426643

Bug: chromium:1132088, v8:10893
Change-Id: I0f34ebc003b220f07df4ecdbf69ea6c06ac1f66a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450016Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70363}