LibWeb: Rename Layout::InitialContainingBlock to Layout::Viewport

The name "initial containing block" was wrong for this, as it doesn't
correspond to the HTML element, and that's specifically what it's
supposed to do! :^)

Userland/Libraries/LibWeb/DOM/Range.cpp

@@ -17,7 +17,7 @@
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/Geometry/DOMRect.h>
 #include <LibWeb/HTML/Window.h>
-#include <LibWeb/Layout/InitialContainingBlock.h>
+#include <LibWeb/Layout/Viewport.h>
 
 namespace Web::DOM {