Sometimes users are trying to tap "PAIR" on WebBluetooth
chooser dialogs without selecting an item. This CL changes
the button to use grey background when no item is selected
and use blue background when an item is selected. This
makes it easier to see if the button is faded out or not.

I attached two screenshots of the chooser with grey and
blue background button in the issue link.

BUG=623117

Review-Url: https://codereview.chromium.org/2107583002
Cr-Commit-Position: refs/heads/master@{#402346}