RFR: Update double-conversion to version 3.1.5

[Hannes Wallnöfer]

Please review the update of the Java port of the V8 double conversion library to the most recent release.

JBS: https://bugs.openjdk.java.net/browse/JDK-8227391
Webrev: http://cr.openjdk.java.net/~hannesw/8227391/webrev.00/

Although the commit log between the two releases is very long [1] (I’m not sure the list is totally accurate since they are on different branches) the actual changes to the code are quite limited. The majority of changes either pertains to the code layout, build system, or are C++ specific and do not apply to the Java port. In the end it amounts to two fixes plus tests along with some code cleanup.

[1]: https://github.com/google/double-conversion/compare/57b1e09...5fa81e8

This will go into 13 or 14, depending on whether the late enhancement is allowed.

Thanks,
Hannes