Add features (feature flags) for V8 serialization experiments.
Add features (feature flags) for V8 serialization experiments.

The intent is to evaluate these w/ Finch, starting with M51.

Context on those two features: crbug.com/594551, crrev.com/1804433004

BUG=chromium:594551

Review URL: https://codereview.chromium.org/1864063002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385746}

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