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