Bad assert in VirtualizingStackPanel.EnsureScrollInfo

* .NET Core Version: all
* Windows version: all
* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes
* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? no

**Problem description:**
The following markup causes a Debug.Assert in `VirtualizingStackPanel.EnsureScrollInfo`

```





```

**Actual behavior:**
When using debug WPF bits, this asserts and either displays the assert dialog or simply crashes (depending on external factors). The assertion occurs when setting both Can*Scroll properties before the VSP is hooked up to a ScrollViewer.

**Expected behavior:**
No assert. The markup is legal.

**Minimal repro:**
[WpfApp5.zip](https://github.com/dotnet/wpf/files/6480889/WpfApp5.zip)

**Notes:**
The markup is actually useless. Instead, you should set the \*ScrollBarVisibility properties on the ScrollViewer. The ScrollViewer uses these properties to set the VSP's Can*Scroll properties, overriding the values from the ItemsPanelTemplate.

```

```

Nevertheless, the assertion should be removed. It's not valid, and it interferes with running an app that may have used the quoted markup. For example, Visual Studio 16.10.0 Preview 2.0.