Improve readability of flow tree dump

Use the PrintTree utility to improve the readability of flow tree
dumps. Blocks and fragments are now split over two dump levels, because
otherwise they are impenetrable. Also start printing the restyle damage of
fragments.

components/layout/block.rs

@@ -62,6 +62,7 @@ use style::values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto};
 use util::geometry::MAX_RECT;
 use util::logical_geometry::{LogicalPoint, LogicalRect, LogicalSize, WritingMode};
 use util::opts;
+use util::print_tree::PrintTree;
 
 /// Information specific to floated blocks.
 #[derive(Clone, RustcEncodable)]
@@ -2035,15 +2036,18 @@ impl Flow for BlockFlow {
     fn mutate_fragments(&mut self, mutator: &mut FnMut(&mut Fragment)) {
         (*mutator)(&mut self.fragment)
     }
+
+    fn print_extra_flow_children(&self, print_tree: &mut PrintTree) {
+        print_tree.add_item(format!("↑↑ Fragment for block: {:?}", self.fragment));
+    }
 }
 
 impl fmt::Debug for BlockFlow {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f,
-               "{:?} - {:x}: frag={:?} ({:?})",
+               "{:?}({:x}) {:?}",
                self.class(),
                self.base.debug_id(),
-               self.fragment,
                self.base)
     }
 }