LibWeb: Rename CSS::StyleResolver => StyleComputer

Resolved style is a spec concept that refers to the weird mix of
computed style and used style reflected by getComputedStyle().

The purpose of this class is to produce the *computed* style for a given
element, so let's call it StyleComputer.

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -23,7 +23,7 @@ HTMLIFrameElement::~HTMLIFrameElement()
 
 RefPtr<Layout::Node> HTMLIFrameElement::create_layout_node()
 {
-    auto style = document().style_resolver().resolve_style(*this);
+    auto style = document().style_computer().compute_style(*this);
     return adopt_ref(*new Layout::FrameBox(document(), *this, move(style)));
 }