Roll engine 3f52888b3bcf..a50f1ef56a05 (4 commits) (#48382)

https://github.com/flutter/engine/compare/3f52888b3bcf...a50f1ef56a05

git log 3f52888b3bcf..a50f1ef56a05 --first-parent --oneline
2020-01-08 gw280@google.com Migrate flutter_runner from flutter_runner::{Thread,Loop} to fml::{Thread,MessageLoop} (flutter/engine#15118)
2020-01-07 ferhat@gmail.com Refactor BitmapCanvas, lazily allocate canvas, fix image composition bug. (flutter/engine#15153)
2020-01-07 ferhat@gmail.com Recover when browser throws on ImageElement.decode due to too many images (flutter/engine#15160)
2020-01-07 garyq@google.com Fix RectHeightStyle::kMax ascent computation bug (flutter/engine#15106)


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

bin/internal/engine.version

-3f52888b3bcf7adb9bc8d7be01f717a0c15b6618
+a50f1ef56a05999bfa97a777cca14fd7a00e8454