First iteration on custom layout.

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 58ed24012c6c3d3d27193b334e330188aa3390a1..8dc9f49de773c7e23595e15c0de7d8d51baf4743 100644
--- a/sky/engine/core/core.gni
+++ b/sky/engine/core/core.gni
@@ -905,6 +905,7 @@ sky_core_files = [
   "inspector/ScriptAsyncCallStack.cpp",
   "inspector/ScriptAsyncCallStack.h",
   "inspector/ScriptGCEventListener.h",
+  "layout/LayoutCallback.h",
   "loader/DocumentLoadTiming.cpp",
   "loader/DocumentLoadTiming.h",
   "loader/EmptyClients.cpp",
@@ -1003,6 +1004,8 @@ sky_core_files = [
   "rendering/RenderBox.h",
   "rendering/RenderBoxModelObject.cpp",
   "rendering/RenderBoxModelObject.h",
+  "rendering/RenderCustomLayout.cpp",
+  "rendering/RenderCustomLayout.h",
   "rendering/RenderFlexibleBox.cpp",
   "rendering/RenderFlexibleBox.h",
   "rendering/RenderGeometryMap.cpp",
@@ -1242,6 +1245,7 @@ core_idl_files = get_path_info([
                                  "html/ImageData.idl",
                                  "html/TextMetrics.idl",
                                  "html/VoidCallback.idl",
+                                 "layout/LayoutCallback.idl",
                                  "painting/Paint.idl",
                                  "painting/PaintingCallback.idl",
                                  "painting/PaintingContext.idl",