Integration tests currently returns a Skia raster image.
This is fine for Flutter widgets, but it misses platform views.

As a result, customer: money or dream cannot test platform views.
Plugins like google_mobile_ads cannot have their own driver tests
that ensure that the native view is sized properly, etc...

We also happen to have the scenario app in the engine, which has
a workaround for this limitation. This workaround breaks occasionally,
and doesn't work with Skia Gold.

For all of these issues, I propose adding a new API to Flutter drive that takes the
device's screenshot and crops the status or action bars if needed
(This will be required on Android if adb is used).

cc @dnfield, @xster, @jmagman for iOS too.

Related bug: http://b/180715355