Fix layout of Create Account / Login when rendering Wikipedia.

When calculating the preferred width for a block, accumulate
the left and right float widths of children separately, which
is then max'ed with the normal flow widths later on.

Ref bug #2554 - improves the layout of the top bar.

components/layout/flow.rs

@@ -61,7 +61,7 @@ use std::iter::Zip;
 use std::raw;
 use std::sync::atomics::{AtomicUint, Relaxed, SeqCst};
 use std::slice::MutItems;
-use style::computed_values::{clear, position, text_align};
+use style::computed_values::{clear, float, position, text_align};
 
 /// Virtual methods that make up a float context.
 ///
@@ -195,6 +195,10 @@ pub trait Flow: fmt::Show + ToString + Share {
         clear::none
     }
 
+    fn float_kind(&self) -> float::T {
+        float::none
+    }
+
     /// Returns true if this float is a block formatting context and false otherwise. The default
     /// implementation returns false.
     fn is_block_formatting_context(&self, _only_impactable_by_floats: bool) -> bool {