Add finalizer to release any unreleased native memory

Summary: Ensure all native memory is released once gc deallocates the java object reference.

Reviewed By: lucasr

Differential Revision: D3992759

fbshipit-source-id: f648b72ead5bdb7257a5197496b19795f71f3788

java/com/facebook/csslayout/CSSNodeJNI.java

@@ -60,6 +60,10 @@ public class CSSNodeJNI implements CSSNodeAPI<CSSNodeJNI> {
       throw new IllegalStateException("You should not reset an attached CSSNode");
     }
 
+    free();
+  }
+
+  private void free() {
     jni_CSSNodeFree(mNativePointer);
     mNativePointer = 0;
     mChildren = null;
@@ -67,6 +71,17 @@ public class CSSNodeJNI implements CSSNodeAPI<CSSNodeJNI> {
     mMeasureFunction = null;
   }
 
+  @Override
+  protected void finalize() throws Throwable {
+    try {
+      if (mNativePointer != 0) {
+        free();
+      }
+    } finally {
+      super.finalize();
+    }
+  }
+
   @Override
   public int getChildCount() {
     return mChildren.size();