Read share targets from prefstore, and filter by engagement.

New function, GetTargetsWithSufficientEngagement, called when browser
receives Share from renderer. Function collates all share targets and
their templates, and keeps targets that have a high enough engagement
score with the user.

Calls picker UI with this filtered list of targets.

BUG=668389

Review-Url: https://codereview.chromium.org/2664033002
Cr-Commit-Position: refs/heads/master@{#449911}
3 files changed