function customDefaultsMerge(objValue, srcValue, key, object, source, stack) {
      if (isObject(objValue) && isObject(srcValue)) {
        // Recursively merge objects and arrays (susceptible to call stack limits).
        stack.set(srcValue, objValue);
        baseMerge(objValue, srcValue, undefined, customDefaultsMerge, stack);
        stack['delete'](srcValue);
      }
      return objValue;
    }
Used by `_.defaultsDeep` to customize its `_.merge` use to merge source
objects into destination objects that are passed thru.

@private
@param {*} objValue The destination value.
@param {*} srcValue The source value.
@param {string} key The key of the property to merge.
@param {Object} object The parent object of `objValue`.
@param {Object} source The parent object of `srcValue`.
@param {Object} [stack] Tracks traversed source values and their merged
 counterparts.
@returns {*} Returns the value to assign.