Increase priority of the EndTracing task (fixes issue #3288)

Use a priority that is guaranteed to execute in a reasonable amount of time.

libcef/browser/trace_subscriber.cc

@@ -119,7 +119,8 @@ bool CefTraceSubscriber::EndTracing(const base::FilePath& tracing_file,
 
   TracingController::GetInstance()->StopTracing(
       TracingController::CreateFileEndpoint(tracing_file,
-                                            std::move(result_callback)));
+                                            std::move(result_callback),
+                                            base::TaskPriority::USER_VISIBLE));
   return true;
 }