Measuring elapsed time

To make it easier to measure how long operations take, we have helpers for C++. These helpers record the elapsed time into histograms, so you have to create suitable Histograms for them first.

API:

void AccumulateTimeDelta(HistogramID id, TimeStamp start, TimeStamp end = TimeStamp::Now());
void AccumulateTimeDelta(HistogramID id, const nsCString& key, TimeStamp start, TimeStamp end = TimeStamp::Now());