Suitably scary warnings attached, as this will yet evolve (notably to handle
host objects, which are not currently handled).

Unit tests adjusted to use the public version of ValueSerializer, eliminating
any need they have to access v8::internal.

With this, Blink can begin using this code experimentally behind a flag as it
develops.

BUG=chromium:148757

Review-Url: https://codereview.chromium.org/2274693002
Cr-Commit-Position: refs/heads/master@{#38915}