Add message and Help Center link to the chooser UI for Mac

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

This is a follow-up patch for:
https://codereview.chromium.org/1661063002/

BUG=492204, 583479

Review URL: https://codereview.chromium.org/1665343003

Cr-Commit-Position: refs/heads/master@{#377949}
1 file changed