calculate header height
calculate header height

billba/BotFramework-WebChat,danmarshall/BotFramework-WebChat,danmarshall/BotFramework-WebChat,billba/BotFramework-WebChat,eanders-MS/BotFramework-WebChat,billba/botchat,billba/BotFramework-WebChat,billba/botchat,billba/botchat,eanders-MS/BotFramework-WebChat,eanders-MS/BotFramework-WebChat,danmarshall/BotFramework-WebChat