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/LineBox.cpp

@@ -35,7 +35,7 @@ void LineBox::trim_trailing_whitespace()
 {
     while (!m_fragments.is_empty() && m_fragments.last().is_justifiable_whitespace()) {
         auto fragment = m_fragments.take_last();
-        m_width -= fragment.width().value();
+        m_width -= fragment.width();
     }
 
     if (m_fragments.is_empty())