Add a new distillability model for the new set of page features

This model is for the new set of features derived from
blink::WebDistillabilityFeatures (added in http://crrev.com/1419033004/).

The old features are extracted by JavaScript code in an isolated world,
so it's slow and consumes more memory than we could afford. Even if we
extract the old features in native code, it will still be too slow, so
we switch to this new set of features.

BUG=509869, 471439

Review URL: https://codereview.chromium.org/1405233009

Cr-Commit-Position: refs/heads/master@{#359512}