LibHTML: Rename ComputedStyle to BoxModelMetrics

There was nothing left in ComputedStyle except the box model metrics,
so this patch gives it a more representative name.

Note that style information is fetched directly from StyleProperties,
which is basically the CSS property name/value pairs that apply to
an element.

Libraries/LibHTML/Layout/BoxModelMetrics.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <LibDraw/Size.h>
+#include <LibHTML/CSS/LengthBox.h>
+
+class BoxModelMetrics {
+public:
+    BoxModelMetrics();
+    ~BoxModelMetrics();
+
+    LengthBox& margin() { return m_margin; }
+    LengthBox& padding() { return m_padding; }
+    LengthBox& border() { return m_border; }
+
+    const LengthBox& margin() const { return m_margin; }
+    const LengthBox& padding() const { return m_padding; }
+    const LengthBox& border() const { return m_border; }
+
+    struct PixelBox {
+        int top;
+        int right;
+        int bottom;
+        int left;
+    };
+
+    PixelBox full_margin() const;
+
+private:
+    LengthBox m_margin;
+    LengthBox m_padding;
+    LengthBox m_border;
+};