Currently v8 gc metric does not produce result if there is no gc event.

This can lead to incorrect aggregation of stories. For example:

https://chromeperf.appspot.com/report?sid=1e689c4dcbac0e4d92aa4a5086221d83dbc9b731ed45dd9d1c31fdec6b0d30e1&rev=436028

In the example, there was a change that improved one story (pinterest) so that it does not do GC. This, however, regressed the aggregate because pinterest was not contributing to that anymore.