LibWeb: Move our "default sizing algorithm" function into a public place

Specifically, this is used by the updated algorithm for parsing a sizes
attribute, so we need access to it there.

No behaviour changes.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -99,6 +99,7 @@ set(SOURCES
     CSS/SelectorEngine.cpp
     CSS/Serialize.cpp
     CSS/Size.cpp
+    CSS/Sizing.cpp
     CSS/StyleComputer.cpp
     CSS/StyleInvalidation.cpp
     CSS/StyleProperties.cpp