[Ash] Copy alpha_shape when recreating layers.

Windows that have a shape applied via AppWindow.setShape lose
their shape after being minimized. This is because minimizing
invokes HidingWindowAnimationObserverBase::DetachAndRecreateLayers.

BUG=408737

Review URL: https://codereview.chromium.org/547283003

Cr-Commit-Position: refs/heads/master@{#294283}

ui/compositor/layer.h

@@ -203,6 +203,7 @@ class COMPOSITOR_EXPORT Layer
   void SetBackgroundZoom(float zoom, int inset);
 
   // Set the shape of this layer.
+  SkRegion* alpha_shape() const { return alpha_shape_.get(); }
   void SetAlphaShape(scoped_ptr<SkRegion> region);
 
   // Invert the layer.

ui/compositor/layer_owner.cc

@@ -46,6 +46,9 @@ scoped_ptr<Layer> LayerOwner::RecreateLayer() {
   new_layer->SetFillsBoundsOpaquely(old_layer->fills_bounds_opaquely());
   new_layer->SetFillsBoundsCompletely(old_layer->FillsBoundsCompletely());
   new_layer->SetSubpixelPositionOffset(old_layer->subpixel_position_offset());
+  SkRegion* alpha_shape = old_layer->alpha_shape();
+  if (alpha_shape)
+    new_layer->SetAlphaShape(make_scoped_ptr(new SkRegion(*alpha_shape)));
 
   // Install new layer as a sibling of the old layer, stacked below it.
   if (old_layer->parent()) {