LibWeb+WebContent: Forbid access to underlying type of CSSPixels

Although DistinctNumeric, which is supposed to abstract the underlying
type, was used to represent CSSPixels, we have a whole bunch of places
in the layout code that assume CSSPixels::value() returns a
floating-point type. This assumption makes it difficult to replace the
underlying type in CSSPixels with a non-floating type.

To make it easier to transition CSSPixels to fixed-point math, one step
we can take is to prevent access to the underlying type using value()
and instead use explicit conversions with the to_float(), to_double(),
and to_int() methods.

Userland/Libraries/LibWeb/Layout/InlineFormattingContext.cpp

@@ -205,7 +205,7 @@ void InlineFormattingContext::apply_justification_to_fragments(CSS::TextJustify
         }
     }
 
-    CSSPixels justified_space_width = whitespace_count > 0 ? (excess_horizontal_space_including_whitespace / static_cast<double>(whitespace_count)) : 0;
+    CSSPixels justified_space_width = whitespace_count > 0 ? (excess_horizontal_space_including_whitespace / whitespace_count) : 0;
 
     // This is the amount that each fragment will be offset by. If a whitespace
     // fragment is shorter than the justified space width, it increases to push