Fix regression introduced with r159483.  The RenderScrollbar's parent class had the signature of a virtual changed out from under it.

BUG=297477

Review URL: https://codereview.chromium.org/48733005

git-svn-id: svn://svn.chromium.org/blink/trunk@160742 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clean WebTestDelegate of impurities.

All these 3-sided patches have landed, so we can
return to the reign of pure virtuals.

All Hail Pure Virtuals!

R=eseidel,abarth
BUG=

Review URL: https://codereview.chromium.org/49203004

git-svn-id: svn://svn.chromium.org/blink/trunk@160743 bbb929c8-8fbe-4397-9dbb-9b2b20218538