Client certainly should not send invalid nav control command, however, due to the latency between engine and client, client may have out of date view of nav states, thus result in invalid command.

Engine should discard invalid nav command, rather than crashes.

one case reported by Khushal: crbug.com/616514 #7


[42519:42519:0603/102957:FATAL:navigation_controller_impl.cc(593)] Check failed: false. 
#0 0x00000832139e base::debug::StackTrace::StackTrace()
#1 0x00000836030f logging::LogMessage::~LogMessage()
#2 0x000002d00ebe content::NavigationControllerImpl::GoToIndex()
#3 0x000002d00da8 content::NavigationControllerImpl::GoBack()
#4 0x0000082e4556 blimp::engine::BlimpEngineSession::GoBack()
#5 0x0000082e577f blimp::engine::BlimpEngineSession::ProcessMessage()
#6 0x00000044dd1d blimp::(anonymous namespace)::DispatchProcessMessage()