Make paired clients list scrollable.
I've accumulated lots of pairings through testing various versions that the
list of paired clients is too large to fit in the dialog, and the Close and
Delete All buttons are not clickable. Although I don't anticipate many users
having this problem, this CL makes the list scrollable.
I've also moved the existing CSS rule under css/html/ since it's not needed
for app remoting.
Review URL: https://codereview.chromium.org/
1162263006
Cr-Commit-Position: refs/heads/master@{#332292}