For heavily loaded servers, even 1 second of trace is too large
to process with the trace viewer; using a float64 here allows
fetching /debug/pprof/trace?seconds=0.1.

Change-Id: I286c07abf04f9c1fe594b0e26799bf37f5c734db
Reviewed-on: https://go-review.googlesource.com/21455Reviewed-by: Austin Clements <austin@google.com>