[LayoutNG] Introduces NGPhysicalConstraintSpace and makes NGConstraintSpace a "view".

third_party/WebKit/Source/core/layout/ng/ng_units.h

Index: third_party/WebKit/Source/core/layout/ng/ng_units.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_units.h b/third_party/WebKit/Source/core/layout/ng/ng_units.h
index 2d8ad7e008801f837ca871fdb9ebdb9236f88590..75951f679660a2c09addb3b2ca741e89d6ee038d 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_units.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_units.h
@@ -26,6 +26,10 @@ struct NGLogicalOffset {
 };
 
 struct NGPhysicalSize {
+  NGPhysicalSize() {}
+  NGPhysicalSize(LayoutUnit width, LayoutUnit height)
+      : width(width), height(height) {}
+
   LayoutUnit width;
   LayoutUnit height;
 };