Add SkEventTracer::onExit hook for flushing Change-Id: Ie170e3406b5daa72f45cc2c057766c9b3866e65e Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1186697 Commit-Queue: Michael Ludwig <michaelludwig@google.com> Reviewed-by: Florin Malita <fmalita@google.com>