LibWeb: Implement basic version of CSSOM View's VisualViewport

We got some errors while loading https://twinings.co.uk/ about this
interface missing, and it looked fairly simple so I sketched it out.
Note that I did leave some FIXMEs where it's not clear exactly which
metrics we should be returning.

Meta/gn/secondary/Userland/Libraries/LibWeb/CSS/BUILD.gn

@@ -57,5 +57,6 @@ source_set("CSS") {
     "StyleValue.cpp",
     "Supports.cpp",
     "Time.cpp",
+    "VisualViewport.cpp",
   ]
 }