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>
7 files changed