Fill in pprof time_nanos from profile timestamp

So when you open a profile with pprof, it gives you the time the profile
was taken:

```
$ pprof pprof.profile
...
Time: Jan 18, 2018 at 8:45pm (AEDT)
```

BUG: 202799418
Change-Id: I9953b7f42cc587af4efc95ae3ec7161b9f2173d8
2 files changed