Replace cloneNodeAndAssociate with Element::cloneElementWithChildren

Use the Element::cloneElementWithChildren to do a "straight" clone, and
then post-process it to associate the cloned nodes with their
corresponding elements.
Move the call to removeDisallowedElementsFromSubtree() into the cloning
sequence since it will have nothing to do if there's no target. The
root has already been verified to be "allowed", so the different
starting element makes no difference on the result.

BUG=589682

Review URL: https://codereview.chromium.org/1759553005

Cr-Commit-Position: refs/heads/master@{#379046}
1 file changed