Expander should count only the longest variant message rather than all variants

[GoogleCodeExporter]

Expander currently adds up the entire length of all message fragments it sees.  
However, some of these are alternate forms of part/all of the message under a 
VariantFragment, which leads to the expansion being overly large.

Intead, it should keep track of VariantFragment structure and remember the 
longest such fragment in computing the total length.

Original issue reported on code.google.com by jat@google.com on 14 Jun 2011 at 10:22

[GoogleCodeExporter]

Original comment by jat@jaet.org on 9 Mar 2013 at 1:50