time: use 1e9 rather than 1e-9 in Duration calculations

time: use 1e9 rather than 1e-9 in Duration calculations

1e-9 has a 1 in the last place, causing some Duration calculations to
have unnecessary rounding errors.  1e9 does not, so use that instead.

Change-Id: I96334a2c47e7a014b532eb4b8a3ef9550e7ed057
Reviewed-on: https://go-review.googlesource.com/33116
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>