This declares v8_enable_slow_dchecks and v8_optimized_debug
as gn args. It adds support for dcheck_always_on and
debugging with v8_optimized_debug = false.

BUG=chromium:474921
NOTRY=true

Review-Url: https://codereview.chromium.org/2024833002
Cr-Commit-Position: refs/heads/master@{#36789}