Async TTM: Clone frozen incoming tokens if necessary.

* transformTokens code sets eltIndex property on token chunks
  pushed on the work-stack for efficiently processing them.
  However, the incoming token array might be frozen which means
  it has to be cloned before the property can be set.

* No change in parser test results.

* Fixes crashers in local client-server RT testing.

Change-Id: Iafa18189a6f8312f7e20a050d22e6c3f017a8679