goprotobuf: Update build tags to use unsafe (but faster) code on Managed VMs.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/91040044

proto/pointer_reflect.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build appengine
+// +build appengine,!appenginevm
 
 // This file contains an implementation of proto field accesses using package reflect.
 // It is slower than the code in pointer_unsafe.go but it avoids package unsafe and can

proto/pointer_unsafe.go

@@ -29,7 +29,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// +build !appengine
+// +build !appengine appenginevm
 
 // This file contains the implementation of the proto field accesses using package unsafe.