LibHTML: Make "white-space: pre" kinda work

This is not ideal, as refusing to break lines will easily overflow the
containing block, and we have no way of dealing with overflow yet.
But as long as you provide enough width, it does look nice. :^)

Libraries/LibHTML/Layout/LayoutText.cpp

@@ -157,10 +157,14 @@ void LayoutText::for_each_source_line(Callback callback) const
     };
 
     for (auto it = view.begin(); it != view.end();) {
-        if (*it == '\n')
+        bool did_commit = false;
+        if (*it == '\n') {
             commit_line(it);
+            did_commit = true;
+        }
         ++it;
-        start_of_line = it;
+        if (did_commit)
+            start_of_line = it;
     }
     if (start_of_line != view.end())
         commit_line(view.end());