We had previously flagged some RenderLayers as being composited, but
paint into their ancestors. Really, they weren't composited, but we
wanted to create the associated backing and its GraphicsLayers in order
to accomplish the following:
 - Clipping (e.g., overflow: hidden).
 - Setting preserves-3d.
 - Applying a perspective transform.

The clipping can be conveniently taken care of via the child layers'
ancestor clipping layers, and the other two tasks can be handled rather
simply with a new graphics layer in the mapping: an ancestor transform
layer. This layer is the owner of the perspective transform.

I'd thought there was one gotcha with using an ancestor transform
layer. We may have nested perspective transforms, and the bounds of the
layer plays a role in its application (it factors into the positioning
of the perspective origin), but we can only have one set of bounds for
the ancestor transform layer. The solution I came up with was to have
zero bounds for this layer, and to express the various perspective
origins with respect to the origin of the compositing ancestor in which
the layers paint.

..but as it turned out, this was not an issue at all: we only respect
the inner-most perspective transform. I've added a layout test to cover
this case and checked that the behavior is the same with this patch.

BUG=None

Review URL: https://codereview.chromium.org/74793012

git-svn-id: svn://svn.chromium.org/blink/trunk@162392 bbb929c8-8fbe-4397-9dbb-9b2b20218538