Decouple Canvas from DisplayList and map Picture and PictureRecorder more directly to their Skia co…

sky/engine/core/painting/Canvas.idl

Index: sky/engine/core/painting/Canvas.idl
diff --git a/sky/engine/core/painting/Canvas.idl b/sky/engine/core/painting/Canvas.idl
index 538e492905fec0d317325082afcfbefe3e73898f..f4d6331e70d259fb6cab65338311f5cbdafc0d20 100644
--- a/sky/engine/core/painting/Canvas.idl
+++ b/sky/engine/core/painting/Canvas.idl
@@ -3,12 +3,9 @@
 // found in the LICENSE file.
 
 // TODO(mpcomplete): Figure out a better SkMatrix representation.
-interface Canvas {
-  // Height and width are used for culling optimizations and do not necessarily
-  // imply that the Canvas is backed by a buffer with any specific bounds.
-  readonly attribute float height;
-  readonly attribute float width;
-
+[
+  Constructor(Canvas canvas),
+] interface Canvas {
   void save();
   // TODO(jackson): Paint should be optional, but making it optional causes crash
   void saveLayer(Rect bounds, /* optional */ Paint paint);