LibWeb: Speed up computed style calculation

Rather than destroying and rebuilding the entire document layout tree in
every call to `ComputedCSSStyleDeclaration::property()`, we now just
make sure that the layout tree exists.

This speeds up the DOM Inspector significantly, from taking several
seconds to select an element, to almost instant. :^)

Userland/Libraries/LibWeb/DOM/Document.h

@@ -144,6 +144,7 @@ public:
 
     void force_layout();
     void invalidate_layout();
+    void ensure_layout();
 
     void update_style();
     void update_layout();