Auto merge of #300 - glennw:demo-fixes, r=pcwalton
Auto merge of #300 - glennw:demo-fixes, r=pcwalton

Fix scrolling on DDG HTML results page.

There are a number of fixes / bandaids here:
 - get_scroll_layer() only returns a layer that is actually scrollable.
 - update the viewport transform when scrolling.
 - propagate the scrolling offset to scrollable child layers.

servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender,servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender