Make reftests work for sky.

-Add a --testing flag to sky_viewer and cause it to paint into an
SkBitmap instead of a ganesh surface so we can get the pixels out.
-Add GetPixelsForTesting to layer.cc to actually grab out the pixels.
-Add a reftest and a mismatch reftest. They need a setTimeout after
the load event. Unclear why or what the right fix is. Maybe we should
give internals some way to force the paint? If we don't have the
setTimeout, we paint a white page (so we do a paint, but with no
content).
-Add a DisplayDelegate to Layer so that Viewer can decide whether
to use the real ganesh backend or the SkBitmap one without littering
the whole code-base with is_testing bools and logic.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/797063002