Don't benchmark so many sizes during the race builder's benchmark run.

This package doesn't even use goroutines.

Cuts off 10 seconds.

Updates #17104

Change-Id: Ibb2c7272c18b9014a775949c656a5b930f197cd4
Reviewed-on: https://go-review.googlesource.com/29158Reviewed-by: David Crawshaw <crawshaw@golang.org>