Stateline's casinos had a merry Christmas as they posted their best December since 2007, bringing in $20.9 million for a 57 percent increase.Plenty of snow ensured there were lots of people at Lake Tahoe to visit the casinos, which are the chief source of Douglas County's gaming revenue.The increase brought the Stateline total to $124.2 million since the fiscal year began on July 1, 2012, up 5.8 percent from last year's total for the same time.Casinos in the East Fork Township and Carson City didn't do nearly as well, posting a 4.47 percent decrease compared to December 2011. Those casinos, …