RFR(JDK 13/java.xml) 8220818: Validator does not find missing match for keyref error

[Joe Wang]

Please review a fix for a regression introduced during JDK 9 
development. The issue was that a patch ported to the JDK added a 
counter to count the number of values found in the XML document, and 
then compared with the constraint's field count. The problem was that 
the counter would get updated as the validator parsed through the XML 
document. When the XML contains multiple elements, the counter reflected 
the last one. If the last one didn't have the problem field, then errors 
in previous elements wouldn't get reported. The fix is to add a field to 
indicate whether the field exists in any of the elements.

All tests and the new test that covers both cases in the original patch 
and that reported in the JBS passed.

JBS: https://bugs.openjdk.java.net/browse/JDK-8220818
webrevs: http://cr.openjdk.java.net/~joehw/jdk13/8220818/webrev/

Thanks,
Joe