WriteFiles windowed writes don't work with session windows

See https://stackoverflow.com/questions/46983318/writing-via-textio-write-with-sessions-windowing-raises-groupbykey-consumption-e

Imported from Jira [BEAM-3122](https://issues.apache.org/jira/browse/BEAM-3122). Original Jira may contain additional context.
Reported by: jkff.

Start with the linked Stack Overflow report and the WriteFiles behavior for session windows; the issue does not name a source file or test. Reproduce the reported GroupByKey consumption error and consult the linked BEAM-3122 Jira entry for any missing context. Done means WriteFiles works with session-windowed input without that failure.

Written by the indexing model from the issue text.