Adding a parameter for previews NQE triggering threshold

This paramater allows triggering previews at different
EffectiveConnectionTypes. Currently, previews are triggered only on
SLOW_2G, but in the future we may wish to change it to 2G without
needing to wait for release cycles. This change also adds some basic
testing around all of the previews params.

BUG=687757

Review-Url: https://codereview.chromium.org/2667243003
Cr-Commit-Position: refs/heads/master@{#447693}
4 files changed