* Remove usages of StackTraceMapper

This class will be replaced with a function type in `test_api`, clean up
usages ahead of time.

In `build_script.dart` the only reference to `StackTraceMapper` is in
the `browser_test.dart` file, but since nothing would call
`setStackTraceMapper` from that import it should be safe to remove the
definition. After reading a serialized mapper from the
`test.browser.mapper` channel, it was ignored.

In `flutter_web_platform.dart` we only were reading from `_mappers`,
never putting values in, so we can remove it entirely. The `load()`
function was never called with a `mapper` argument.

This reverts commit 3aa7a800.

This reverts commit 868b0e6d.

provide Guidance for MultiChildRenderObjectWidget implementations

* Revert "Revert "install goldctl in docker build (#46640)" (#50088)"

This reverts commit 0299398d.

This is to remove the flakiness associated with running those tests on
ios/13 when they always fail.

Context:
https://github.com/flutter/flutter/issues/49635

This reverts commit c2353b29.

This reverts commit e03f4391.

This step is now executed on the LUCI bot.

81dffd124 Roll src/third_party/skia af99f3ee6017..71ce449d2814 (19 commits) (flutter/engine#16356) (#50119)

https://github.com/flutter/engine/compare/44f24bd9807f...f44f50f3c587

git log 44f24bd9807f..f44f50f3c587 --first-parent --oneline
2020-02-04 chinmaygarde@google.com Fix race in SkiaGPUObject unit-tests. (flutter/engine#16351)
2020-02-04 matthew-carroll@users.noreply.github.com Added API guards to flags in PlatformPlugin that were introduced after Flutter's min SDK version. (flutter/engine#13965)


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 chinmaygarde@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