Refactor ChooserBubbleUiController on Mac

This is part 2 of enabling the chooser UIs for Bluetooth from
Apps to enable developers to make a transition.

Since the chooser content view is the same for WebBluetooth on desktop
and Chrome Apps, it is better to pull it out of ChooserBubbleUiController
and have a class ChooserContentView that can be reused.

BUG=612942, 577170

Review-Url: https://codereview.chromium.org/1995793002
Cr-Commit-Position: refs/heads/master@{#396494}
4 files changed