Called after all processes have sent their trace data. |tracing_file| is the path at which tracing data was written. The client is responsible for deleting |tracing_file|.
Start tracing events on all processes. Tracing is initialized asynchronously and |callback| will be executed on the UI thread after initialization is complete.
If CefBeginTracing was called previously, or if a CefEndTracingAsync call is
Base structure.