Clip Scrollbar painting like contents painting in ScrollView

Previously, it was possible to paint a scroll corner that was larger
than the iframe it was attached to, effectively enabling it to paint
over parts of the parent frame. This change clips scrollbar painting
the same way we clip contents.

BUG=331168

Review URL: https://codereview.chromium.org/220243010

git-svn-id: svn://svn.chromium.org/blink/trunk@170625 bbb929c8-8fbe-4397-9dbb-9b2b20218538