Remove unncessary Scrollbar.h include which will allow me to move all of ScrollbarTheme* to Source/platform

BUG=297477

Review URL: https://codereview.chromium.org/26558002

git-svn-id: svn://svn.chromium.org/blink/trunk@159238 bbb929c8-8fbe-4397-9dbb-9b2b20218538