@Override
    protected int getByteOffset() {
        return offset;
    }
Takes ownership of the given byte array. And keeps note of where
the interesting utf8 sequence start and its length.
@param utf8data The backing byte array.
@param offset   The start of the utf8 sequence.
@param bytes    The length of the utf8 sequence.
vespajlib/src/main/java/com/yahoo/text/Utf8PartialArray.java