This change allows code to be sent safely to a partially sandboxed playground using the Native Client environment.
Execution of non-Go code is blocked when the RunScripts is false to prevent inclusion of code that might escape by virtue of being under a different runtime environment.
Two options for communicating whether to prevent non-Go code were considered: as has been done here and alternatively, using a message field. The latter was not chosen to close that as an attack option.
Another will be follow that adds a -nacl flag to the present command to allow sandboxing of playground code in presentations.
See discussion here: https://groups.google.com/d/topic/golang-dev/Hy-7PBP-T4Q/
LGTM=adg
R=adg, dave
CC=golang-codereviews
https://golang.org/cl/74740045
