LibWeb: Convert BorderData::width to CSSPixels

The `clip_shrink` optimization in `paint_background()` now also
correctly uses DevicePixels, instead of reducing a DevicePixel rect by
a CSSPixels amount.

Userland/Libraries/LibWeb/CSS/ComputedValues.h

@@ -155,7 +155,7 @@ struct BorderData {
 public:
     Color color { Color::Transparent };
     CSS::LineStyle line_style { CSS::LineStyle::None };
-    double width { 0 };
+    CSSPixels width { 0 };
 
     bool operator==(BorderData const&) const = default;
 };