[15] RFR: 8187987: Add a mechanism to configure custom variants in HijrahChronology

[naoto.sato at oracle.com]

Hi,

Please review the fix to the following issue:

https://bugs.openjdk.java.net/browse/JDK-8187987

The proposed CSR and changeset are located at:

CSR: https://bugs.openjdk.java.net/browse/JDK-8236810
Webrev: https://cr.openjdk.java.net/~naoto/8187987/webrev.00/

The spec of java.time.chrono.HijrahChronology suggests that it could 
load custom variants of Hijirah calendar type using properties files. 
However it does not work as designed. This change intends to make it work.

Naoto