Summary:
This PR fixes issue #295
- Created a new internal YGNodeHandle class extending SafeHandle.
- YogaNode now stores a reference to YGNodeHandle.
- Removed finalizer from YogaNode.
- Pulling in System.Runtime.Handles 4.3.0.
Closes https://github.com/facebook/yoga/pull/297
Reviewed By: emilsjolander
Differential Revision: D4365462
Pulled By: splhack
fbshipit-source-id: 73293b105962cbedb71f4e3d8d1244251db1ea79
4717594e93