GitLab

Remove chromium defaults for v8_optimized_debug and
v8_use_external_startup_data.

This is not needed after v8 provides these defaults:
https://codereview.chromium.org/2025803003/
https://codereview.chromium.org/2024833002/

It also interferes if somebody tries to override the gn args
with a different value.

BUG=chromium:616034
TBR=alokp@chromium.org, brettw@chromium.org

Review-Url: https://codereview.chromium.org/2058033002
Cr-Commit-Position: refs/heads/master@{#406067}