[Prerender] Get the prerender mode from Finch field trial.

This CL introduces a new NoStatePrefetch feature, controlled by Finch.

The feature can be disabled, and then there is no prerender or prefetch happening.

It can be enabled with 3 modes:
- prerender (default for now)
- prefetch
- simple load.

BUG=668997
TBR=pasko

Review-Url: https://codereview.chromium.org/2537503002
Cr-Commit-Position: refs/heads/master@{#436932}
18 files changed