Wire up OpacityLayer to Scenic

[mikejurka]

A previous version of this change also removed
system compositing of PhysicalShapeLayers on
Fuchsia. In this reland, keep using system
composting for PhysicalShapeLayers.

[mikejurka]

This should be identical to ea67e5b except without the changes to PhysicalShapeLayer, and one merge conflict when applying this to master.

This line was added to OpacityLayer::Preroll by Jim in 8595361, and I added it to this CL too:

Layer::AutoPrerollSaveLayerState save =
      Layer::AutoPrerollSaveLayerState::Create(context);

[iskakaushik]

LGTM

[flar]

I didn't have time to review everything here, but I verified that you maintained the PrerollSaveLayerState code that I added. Thanks!

FWIW, LGTM.

[mikejurka]

I think I figured out the flashing issue. Changed opacity_layer.cc:69 from:

if (OpacityLayerBase::can_system_composite()) {
    set_dimensions(SkRRect::MakeRect(paint_bounds()));
    set_needs_system_composite(true);
  } else {

to:

if (OpacityLayerBase::can_system_composite() && needs_system_composite()) {
    set_dimensions(SkRRect::MakeRect(paint_bounds()));
  } else {