Remove ScrollView::platform*, which do nothing.

These methods are no-ops and have no meaningful existence anymore.

BUG=260522

Review URL: https://chromiumcodereview.appspot.com/19328002

git-svn-id: svn://svn.chromium.org/blink/trunk@154333 bbb929c8-8fbe-4397-9dbb-9b2b20218538