Change the expand button to unfold-more.

This isn't awesome, but it was the best icon I could find and it
at least will be more clear since it's not using something that's
overloaded for opening menus.

BUG=404049
NOTRY=true

Review URL: https://codereview.chromium.org/475323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180393 bbb929c8-8fbe-4397-9dbb-9b2b20218538