Clone tokens in Sanitizer:onAnchor before updating them.

* Since original tokens are frozen, and not all of them are
  always cloned (used to be cloned always in AttrExpander till
  that changed in 46119efb), clone them in onAnchor if they
  are going to be modified -- do we need a 'isFrozen' flag on
  tokens which we can test to avoid cloning unnecessarily?

* Fixes: node roundtrip-test.js 'Wikipedia:Importance/Archive 01'
  which used to crash with an attempt to set a read only attr.
  of the token being sanitized.

* No change in parserTest results.

Change-Id: Ia29756acb15f9762e399c57efb7687b12fec1c96
Merge "Clone tokens in Sanitizer:onAnchor before updating them."