JDK-8140384: TicksToTimeHelper::milliseconds() should return a double

[Bengt Rutisson]

Hi all,
Could I have  a couple of reviews for this small change?

http://cr.openjdk.java.net/~brutisso/8140384/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8140384

The TicksToTimeHelper class use different return values depending on if 
you want to convert to seconds or milliseconds.

class TicksToTimeHelper : public AllStatic {
 public:
  enum Unit {
    SECONDS = 1,
    MILLISECONDS = 1000
  };
  static double seconds(const Tickspan& span);
  static jlong milliseconds(const Tickspan& span);
};

No one is using the conversion to milliseconds() but because it returns 
a jlong it loses precision.

Making milliseconds() return a double allows for coming uses to get 
millisecond values with higher precision.

Thanks,
Bengt