need microsecond accuracy for HAR timings

All,

The RT API has timings in milliseconds with accuracy to the thousandths of a millisecond. On the other hand the HAR spec has timings only in milliseconds.

This is an issue if we plan to use the value in ResponseEnd to indicate that a preloaded object was used as per https://github.com/igrigorik/resource-hints/issues/19#issuecomment-54350393.

Do we need to specify HAR timings with microsecond accuracy?

Thanks,

Peter