[9] RFR 8079693: Add support for ECDSA P-384 and P-521 curves to XML Signature

[Jason Uh]

Please review this change, which adds support for two curves to XML 
Signature.

webrev: http://cr.openjdk.java.net/~juh/8079693/00/
bug: https://bugs.openjdk.java.net/browse/JDK-8079693

Thanks,
Jason