LibWeb: Teach line layout to collapse whitespace across fragments

This kind of HTML now produces a single piece of whitespace:

<span> </span> <span> </span> <span> </span>

We achieve this by checking if the last fragment on the last line ends
in whitespace. If so, we either don't add a fragment at all (for the
current chunk) or we simply skip over all whitespace at the head of
the current chunk (instead of collapsing it to a single ' '.)

Libraries/LibWeb/Layout/LineBox.cpp

@@ -73,4 +73,11 @@ void LineBox::trim_trailing_whitespace()
     }
 }
 
+bool LineBox::ends_in_whitespace() const
+{
+    if (m_fragments.is_empty())
+        return false;
+    return m_fragments.last().ends_in_whitespace();
+}
+
 }