Also, avoid Region creation when tracing is disabled.
Unfortunate side-effect of this change is that we no longer trace
pre-existing regions in tracing, but we can add the feature in
the future when we find it useful and justifiable. Until then,
let's avoid the overhead from this low-level api use as much as
possible.

goos: linux
goarch: amd64
pkg: runtime/trace

// Trace disabled
BenchmarkStartRegion-12 2000000000	         0.66 ns/op	       0 B/op	       0 allocs/op
BenchmarkNewTask-12    	30000000	        40.4 ns/op	      56 B/op	       2 allocs/op

// Trace enabled, -trace=/dev/null
BenchmarkStartRegion-12  5000000	       287 ns/op	      32 B/op	       1 allocs/op
BenchmarkNewTask-12    	 5000000	       283 ns/op	      56 B/op	       2 allocs/op

Also, skip other tests if tracing is already enabled.

Change-Id: Id3028d60b5642fcab4b09a74fd7d79361a3861e5
Reviewed-on: https://go-review.googlesource.com/109115Reviewed-by: Peter Weinberger <pjw@google.com>