playground: allow CORS requests to /share
playground: allow CORS requests to /share

Update golang/go#21691

Change-Id: I943e2fa260fe5e73407aeabec4d86448817626f9
Reviewed-on: https://go-review.googlesource.com/60690
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

golang/playground,golang/playground,golang/playground