Add an explicit virtual destructor to WebScrollbarBehavior.

This fixes a clang warning on the Chrome side when trying to compile
scoped_ptr<WebScrollbarBehavior> about "delete called on class with virtual
functions but non-virtual destructor".

BUG=337919
TEST=none
TBR=abarth

Review URL: https://codereview.chromium.org/183833005

git-svn-id: svn://svn.chromium.org/blink/trunk@168076 bbb929c8-8fbe-4397-9dbb-9b2b20218538