Introduce PaintingContext, which will be used for custom painting

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 44cb7c58a1ca1482ccf1102df1009cd51c215f22..b3bf1edfd401d9c3734ef476ae0422fe5018e823 100644
--- a/sky/engine/core/core.gni
+++ b/sky/engine/core/core.gni
@@ -940,6 +940,10 @@ sky_core_files = [
   "page/PageVisibilityState.cpp",
   "page/PageVisibilityState.h",
   "page/SpellCheckerClient.h",
+  "painting/Paint.cpp",
+  "painting/Paint.h",
+  "painting/PaintingContext.cpp",
+  "painting/PaintingContext.h",
   "rendering/BidiRun.h",
   "rendering/BidiRunForLine.cpp",
   "rendering/BidiRunForLine.h",
@@ -1234,6 +1238,8 @@ core_idl_files = get_path_info([
                                  "html/ImageData.idl",
                                  "html/TextMetrics.idl",
                                  "html/VoidCallback.idl",
+                                 "painting/Paint.idl",
+                                 "painting/PaintingContext.idl",
                                ],
                                "abspath")