Roll engine ba51408257b6..9f8b1d223ee8 (3 commits) (#38974)

git@github.com:flutter/engine.git/compare/ba51408257b6...9f8b1d223ee8

git log ba51408257b6..9f8b1d223ee8 --no-merges --oneline
2019-08-21 stuartmorgan@google.com Clean up Windows and Linux build output (flutter/engine#11324)
2019-08-21 stuartmorgan@google.com Roll angle licenses (flutter/engine#11035)
2019-08-21 skia-flutter-autoroll@skia.org Roll src/third_party/skia 0b9801eadc12..f99b7960f433 (3 commits) (flutter/engine#11338)

The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff (liyuqian@google.com), and stop
the roller if necessary.

bin/internal/engine.version

-ba51408257b6f261219781ada411f3dcb2da38f8
+9f8b1d223ee8dd5d49667f78433a3c022f07a26c