Add message and Help Center link to the chooser UI

This patch added message "Not seeing your device? Get
help" at the bottom of the chooser UI, and the "Get help"
links to a Help Center URL. The message is added below
the "Connect" and "Cancel" button with a grey separator.

BUG=492204, 583479

Review URL: https://codereview.chromium.org/1661063002

Cr-Commit-Position: refs/heads/master@{#377216}
10 files changed