Add an explicit virtual destructor to WebScrollbarBehavior.
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@168076 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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