Roll engine 28d7900ed767..da84d59f64b1 (2 commits) (#40309)

git@github.com:flutter/engine.git/compare/28d7900ed767...da84d59f64b1

git log 28d7900ed767..da84d59f64b1 --no-merges --oneline
2019-09-12 50503328+flar@users.noreply.github.com Revert "Manage iOS contexts separately (#12078)" (flutter/engine#12233)
2019-09-11 gw@gwright.org.uk Manage iOS contexts separately (flutter/engine#12078)


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

-28d7900ed767fe88228d2d58b0941544e0f1c680
+da84d59f64b104bb5b7086231ffdf8f3ea79072f