Get rid of LayerFragment.

We don't have fragmentation. So no need for this abstraction.
This simplifies a lot of code so that we can do less work.
For example, we can check shouldPaintContent only up in
paintLayerContents instead of in each nested function.

Instead of passing the LayerFragment, pass the layer location
and the clip rect for each nested function.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/789643002