This change makes stateEndTop use isSpace instead of specifically
recreating the same functionality.

Change-Id: I81f8f51682e46e7f8e2b9fed423a968457200625
Reviewed-on: https://go-review.googlesource.com/c/121797Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>