Add back in a backwards-compatible constructor for
FlagRegisterer.

Because this backwards-compatible API is intended to be
short-lived, I did it in the simplest, least invasive way
possible, which involved cutting-and-pasting.

R=ncalvin,jkline
DELTA=27  (27 added, 0 deleted, 0 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=3065


git-svn-id: https://gflags.googlecode.com/svn/trunk@62 6586e3c6-dcc4-952a-343f-ff74eb82781d