UPDATE 5-Chipotle hit with U.S. subpoena over California norovirus outbreak

* Says norovirus contamination in California being probed * Estimates 14.6 pct fall in 4th-qtr same-store sales * Announces $300 million share buyback (Adds details on investigation, datelines, additional byline) By Sarah N. Lynch and Tom Polansek WASHINGTON/CHICAGO, Jan 6 (Reuters) - Chipotle Mexican Grill Inc, under scrutiny for months over outbreaks of foodborne illness across several U.S. states, said on Wednesday it was served with a subpoena in a federal criminal probe li