Explicitly tell Smart Select whether or not to suggest

This is a piece of the larger CL crrev.com/2785853002. Since the goal is to have
the select action mode be triggered by contextmenu events instead of selection
changes we need to have Smart Selection be triggered explicitly instead of through
selection events.

BUG=627234

Review-Url: https://codereview.chromium.org/2863573004
Cr-Commit-Position: refs/heads/master@{#469887}
4 files changed