Remove FramelessScrollView.

This class was only used for rendering <select> elements.  PopupListBox now
inherits ScrollView directly.  (With future refactorings of the scrolling code
it may not even do that.)

PopupContainer now inherits Widget.  There is no need for it to be a ScrollView
since it never actually scrolls.

Review URL: https://codereview.chromium.org/562513004

git-svn-id: svn://svn.chromium.org/blink/trunk@181769 bbb929c8-8fbe-4397-9dbb-9b2b20218538