Remove PaintLayerPaintingOverflowContents.

It was added for compositing overflow:scroll, which we don't do.
This also means that only the RespectOverflowClip enum value is
used. And PaintingClipRectsIgnoringOverflowClip is unused.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/778753002