This sets default values for v8_enable_gdbjit and
v8_imminent_deprecation_warnings, while allowing to override
them as gn args.

Without this patch the project-specific overrides have no
effect.

BUG=chromium:474921, chromium:626064

Review-Url: https://codereview.chromium.org/2153753002
Cr-Commit-Position: refs/heads/master@{#37797}