Roll engine 7d8c6d9..e12decf (2 commits) (flutter#40458)

git@github.com:flutter/engine.git/compare/7d8c6d9c1a5c...e12decf

git log 7d8c6d9..e12decf --no-merges --oneline
2019-09-13 xster@google.com Clean up after AppLifecycleTests (flutter/engine#12273)
2019-09-13 dnfield@google.com Close the tree (flutter/engine#12268)


If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/flutter-engine-flutter-autoroll
Please CC jimgraham@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

bin/internal/engine.version

@@ -1 +1 @@
-7d8c6d9c1a5c1ce5303a5694bc9f49819c772f5f
+e12decfdd210efdd7cc73b4311f0c62bb27c215e