Test that scrollBy works on the visual viewport.

This is a testharness.js-based test.
PASS Scroll x:0, y:1000, smooth:true 
PASS Scroll x:10, y:400, smooth:true 
PASS Scroll x:0, y:1000, smooth:false 
PASS Scroll x:10, y:400, smooth:false 
Harness: the test ran to completion.