code: public int getBoneIndex(GVRSceneObject bone)
      {
          for (int i = 0; i < getNumBones(); ++i)
              if (mBones[i] == bone)
                  return i;
          return -1;
      }
docs: Get the bone index for the given scene object.
  
  @param bone GVRSceneObject bone to search for
  @return bone index or -1 for root bone.
  @see #getParentBoneIndex