Event Tracer Leaking - Skia

The leakTracer argument to SkEventTracer::SetInstance is removed and now behaves as if leakTracer=true. Previously, with leakTracer=false the SkEventTracer would be deleted in an atexit handler, but this could lead to race conditions in a multithreaded application during process exiting. Leaking the object at exit is effectively the same behavior but avoids the races.
SkEventTracer adds a new onExit() virtual that defaults to doing nothing. This will be called on process exit and can be used in place of any flushing logic in a destructor.