Use runtime.GOROOT instead of os.Getenv("GOROOT") to reference
trace-viewer html file. GOROOT env var is not necessary set,
runtime.GOROOT has a default value for such case.

Change-Id: I906a720f6822915bd9575756e6cbf6d622857c2b
Reviewed-on: https://go-review.googlesource.com/13593Reviewed-by: Russ Cox <rsc@golang.org>