Add tutorial for performance debugging / profiling.

[davidsoergel]

From https://groups.google.com/a/tensorflow.org/forum/#!topic/tfjs/_IDVt3wQFXA:

What are best practices in timing operations that involve data transfer between CPU/GPU to identify bottlenecks?

[davidsoergel]

It might be sufficient to update https://deeplearnjs.org/docs/tutorials/performance.html and move it to the new TF.js site.

[caisq]

cc @dsmilkov

[rthadur]

This has been included here https://www.tensorflow.org/js/guide/platform_environment . Closing this request.