Auto merge of #13401 - notriddle:master, r=pcwalton

Implement sequential fallback to float speculation

This shouldn't impact any pages that are already rendering correctly, but it is a very naive implementation of this pass.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13284 and fix #13223
- [X] There are tests for these changes

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components/layout/flow.rs

@@ -238,7 +238,8 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
     /// it as laid out by its parent.
     fn assign_block_size_for_inorder_child_if_necessary<'a>(&mut self,
                                                             layout_context: &'a LayoutContext<'a>,
-                                                            parent_thread_id: u8)
+                                                            parent_thread_id: u8,
+                                                            _content_box: LogicalRect<Au>)
                                                             -> bool {
         let might_have_floats_in_or_out = base(self).might_have_floats_in() ||
             base(self).might_have_floats_out();