Summary:
- Update CSSNodeFree to unlink parent and children for C#, Java and Objective-C bindings finalizer.
- [C#] Fix build (Fix#232)
- [C#] Add Clear API for convenience as CSSNodeFreeRecursive.
- [C#] Revise and add unit tests
Reviewed By: emilsjolander
Differential Revision: D4069655
fbshipit-source-id: 1fd764059784d7968af38b6aaf7fb6f70fdee8ee
2168d68007