Auto merge of #8087 - asajeffrey:dont_dirty_twice, r=pcwalton
Auto merge of #8087 - asajeffrey:dont_dirty_twice, r=pcwalton

Removced unncessary call to dirty() in Document::node_and_heritage_ch…

Removed unncessary call to dirty() in Document::node_and_heritage_change, since the node is aready dirtied by force_dirty_ancestors().

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