Optionally use new GN optimization config.

Optionally use new GN optimization config.

The GN default optimization flag is changing from being specific to the
debug/release status of the build to always being "default_optimization" so
it's easier to override without being conditional on the exact setup in
BUILDCONFIG.gn. See https://codereview.chromium.org/1324623005/

Since V8 is DEPS-ed in, it will need to support both modes to allow a landing.
This patch uses a temporary transitional flag I added to BUILDCONFIG to
indicate which variant should be used. After the patch is landed, we can remove
the new conditions and just remove default_optimization.

Review URL: https://codereview.chromium.org/1333963002

Cr-Commit-Position: refs/heads/master@{#30703}