Summary:
- Unused _context GCHandle prevent calling CSSNode destructor.
- CSSNode C# impl will retain children node when calling CSSNodeInsertChild, so the lifetime of C# object is the same as the lifetime of native instance.
Reviewed By: emilsjolander
Differential Revision: D3981697
fbshipit-source-id: 71f0f7be97fd694d27934d7d07deb64ee08b2840