Always give RenderParts a RenderLayer.

This lays the groundwork for removing scheduleLayerUpdate calls in
RenderLayerCompositor for iframes. Instead of changing the layer
type when the frame is composited, change it so that the layer
type remains ForcedLayer, but the isSelfPainting bit respects
the m_compositing bit on RenderLayerCompositor.

This exposed a bug in RenderBox::nodeAtPoint where it wouldn't
recurse correctly because the child now has a layer, but isn't
self-painting. Before we'd never hit this code path with
a non-self-painting layer.

The rebaselines are all just cases of things getting a layer
where they didn't used to.

Review URL: https://codereview.chromium.org/302083002

git-svn-id: svn://svn.chromium.org/blink/trunk@175436 bbb929c8-8fbe-4397-9dbb-9b2b20218538