@Override
    public int size() {
        return (primary.size() >= secondary.size())
                ? countUnique(primary, secondary)
                : countUnique(secondary, primary);
    }
A read-only map which forwards lookups to a primary map, and then a secondary for
keys not existing in the primary.

@author bratseth
container-search/src/main/java/com/yahoo/search/query/profile/ChainedMap.java