Make it possible to custom-paint without an Element.

sky/engine/core/core.gni

Index: sky/engine/core/core.gni
diff --git a/sky/engine/core/core.gni b/sky/engine/core/core.gni
index 2a78c74f4d9997c3d6039bbd9b448c1a763eab6f..51547c81d7be7eae48791fbe420c86db6a35c20a 100644
--- a/sky/engine/core/core.gni
+++ b/sky/engine/core/core.gni
@@ -833,6 +833,8 @@ sky_core_files = [
   "page/PageVisibilityState.cpp",
   "page/PageVisibilityState.h",
   "page/SpellCheckerClient.h",
+  "painting/Canvas.cpp",
+  "painting/Canvas.h",
   "painting/Paint.cpp",
   "painting/Paint.h",
   "painting/PaintingCallback.cpp",
@@ -841,6 +843,10 @@ sky_core_files = [
   "painting/PaintingContext.h",
   "painting/PaintingTasks.cpp",
   "painting/PaintingTasks.h",
+  "painting/Picture.cpp",
+  "painting/Picture.h",
+  "painting/PictureRecorder.cpp",
+  "painting/PictureRecorder.h",
   "rendering/BidiRun.h",
   "rendering/BidiRunForLine.cpp",
   "rendering/BidiRunForLine.h",
@@ -1095,9 +1101,12 @@ core_idl_files = get_path_info([
                                  "html/TextMetrics.idl",
                                  "html/VoidCallback.idl",
                                  "layout/LayoutCallback.idl",
+                                 "painting/Canvas.idl",
                                  "painting/Paint.idl",
                                  "painting/PaintingCallback.idl",
                                  "painting/PaintingContext.idl",
+                                 "painting/Picture.idl",
+                                 "painting/PictureRecorder.idl",
                                ],
                                "abspath")