Review request for #151 Open VirtualFlow and other relating classes API for sublclassing

[Sam']

Please review the GitHub pull request:
https://github.com/javafxports/openjdk-jfx/pull/163
<https://github.com/javafxports/openjdk-jfx/pull/235>

which fixes:


Open VirtualFlow API for subclassing

https://github.com/javafxports/openjdk-jfx/issues/151
<https://bugs.openjdk.java.net/browse/JDK-8188810>

Thank you,
Samir