Fixed #4170 - Incremental reflow wasn't being aggressive enough when nodes get reparented.

When inserting a node that was already dirtied, the dirtying logic
would short circuit: "This node is already dirty? Great! Then its
parents must be HAS_DIRTY_DESCENDANTS, too! Let's skip that step."

This isn't appropriate when nodes move around the tree. In that case,
the node may be marked HAS_CHANGED, but ancestors may not yet have
the HAS_DIRTY_DESCENDANTS flag set.

This patch adds a `content_and_heritage_changed` hook in the document,
to deal with these cases appropriately.
auto merge of #4194 : cgaebel/servo/incremental-reflow-fix, r=pcwalton

When inserting a node that was already dirtied, the dirtying logic
would short circuit: "This node is already dirty? Great! Then its
parents must be HAS_DIRTY_DESCENDANTS, too! Let's skip that step."

This isn't appropriate when nodes move around the tree. In that case,
the node may be marked HAS_CHANGED, but ancestors may not yet have
the HAS_DIRTY_DESCENDANTS flag set.

This patch adds a `content_and_heritage_changed` hook in the document,
to deal with these cases appropriately.