This PR also wraps up the soft transition for #27417.

Because some unit tests depend on the real Scenic which isn't hermetic, `flutter_runner_tests` and `flutter_runner_scenic_tests` aren't currently compatible with cfv2.
This will change once a FakeScenic is created (https://fxbug.dev/79873).

See https://fxbug.dev/79691.

https://skia.googlesource.com/skia.git/+log/8c3adb8b1adb..fe5d07a8e471

2021-07-16 recipe-mega-autoroller@chops-service-accounts.iam.gserviceaccount.com Roll recipe dependencies (trivial).
2021-07-16 csmartdalton@google.com Work around a Mac compiler bug with "!isinf()"
2021-07-16 johnstiles@google.com Add checkbox to Viewer to force-enable Runtime Blends.

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-autoroll
Please CC johnstiles@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/+doc/main/autoroll/README.md

* Revert "Roll Dart SDK from 87a4768078b2 to 2abf053a8e9a (1 revision) (#27489)"

This reverts commit 04e2b8a0.

* Revert "Roll Dart SDK from 207232b5abe0 to 87a4768078b2 (5 revisions) (#27476)"

This reverts commit 76eb77a7.

* Fixes scrollable behavior in voiceover

* fix crash

This reverts commit 7e252b58.

Ninja runs `action` scripts using the default python interpretter, and
most flutter build scripts that use python currently require python2. In
order to support python3 scripts as well, the `python3_action` invokes a
wrapper python2-based script via GN `action` target, and the wrapper
invokes the python3 interpretter with the given script.

This new target is required to support flutter/engine#26880, but is
broken out to a separate PR (by request) to support other more immediate
use cases.