Roll engine 4d7ea15cae36..bf7796611dc1 (2 commits) (#39340)

git@github.com:flutter/engine.git/compare/4d7ea15cae36...bf7796611dc1

git log 4d7ea15cae36..bf7796611dc1 --no-merges --oneline
2019-08-27 chris@bracken.jp Revert "Reuse texture cache in ios_external_texture_gl. (#9806)" (flutter/engine#11522)
2019-08-27 liujilong.me@gmail.com Reuse texture cache in ios_external_texture_gl. (flutter/engine#9806)


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 garyq@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