Introduce PaintingContext, which will be used for custom painting

sky/engine/core/painting/Paint.cpp

Index: sky/engine/core/painting/Paint.cpp
diff --git a/sky/engine/tonic/dart_class_provider.cc b/sky/engine/core/painting/Paint.cpp
similarity index 65%
copy from sky/engine/tonic/dart_class_provider.cc
copy to sky/engine/core/painting/Paint.cpp
index 70ce8541f3a45e5e20e2339bef186f0fb75110ae..f3e6fcae0fbd241cca373bde147ae83dce270c6c 100644
--- a/sky/engine/tonic/dart_class_provider.cc
+++ b/sky/engine/core/painting/Paint.cpp
@@ -3,11 +3,16 @@
 // found in the LICENSE file.
 
 #include "sky/engine/config.h"
-#include "sky/engine/tonic/dart_class_provider.h"
+#include "sky/engine/core/painting/Paint.h"
 
 namespace blink {
 
-DartClassProvider::~DartClassProvider() {
+Paint::Paint()
+{
 }
 
-}  // namespace blink
+Paint::~Paint()
+{
+}
+
+} // namespace blink