code: final GVRReference findReference(long nativePointer) {
          for (GVRReference reference : mReferenceSet) {
              if (reference.mNativePointer == nativePointer) {
                  return reference;
              }
          }
          return null;
      }
docs: Explicitly close()ing an object is going to be relatively rare - most
  native memory will be freed when the owner-objects are garbage collected.
  Doing a lookup in these rare cases means that we can avoid giving every @link
  {@link GVRHybridObject} a hard reference to its {@link GVRReference}.