Bug 732971 - Create a new layer type for subdocuments with a resolution. r=
Bug 732971 - For Fennec move the resolution from the top-level presShell to the content presShell. r=

The bulk of this patch is fixing up pieces of code that infer the resolution
as the inverse of the scaling transform on the root layer. This used to be
how various bits of gfx code obtained the resolution, but now that we can set
the resolution on the actual presShell that contains the content, we can also
just read the resolution from the FrameMetrics directly.
Bug 732971 - Apply the async transform after the CSS transform for a given layer. r=BenWa

Since the CSS transform includes a 1/resolution scaling factor, the translation
amount in the async transform is now in LayoutDevicePixels instead of LayerPixels.
The shadow tree transform is now effectively:
(1/resolution * rest of CSS transform * async translation * async resolution)
Whereas before it was
(async translation * async resolution * 1/resolution * rest of CSS transform)