LibHTML: Add StyleProperties::line_height()

We currently hard-code the line height to 140% of the font glyph height
and this patch doesn't fix the hard-coding, but at least moves it out
of LayoutText and into StyleProperties where it can be re-used until
the day we go and do a proper implementation of CSS line-height. :^)

Libraries/LibHTML/CSS/StyleProperties.h

@@ -32,6 +32,8 @@ public:
         return *m_font;
     }
 
+    int line_height() const;
+
 private:
     HashMap<unsigned, NonnullRefPtr<StyleValue>> m_property_values;