ref: Split the security.Call and rpc.ServerCall interfaces.

It turns out they don't have that much in common and now that security
functions take context.T there is no reason to have them conjoined.

Change-Id: I9a562d942a9f77c09049632848c55485a4e05514
MultiPart: 2/2
diff --git a/services/identity/internal/blesser/oauth.go b/services/identity/internal/blesser/oauth.go
index e086e40..7dbd413 100644
--- a/services/identity/internal/blesser/oauth.go
+++ b/services/identity/internal/blesser/oauth.go
@@ -68,10 +68,10 @@
 	if err != nil {
 		return noblessings, "", err
 	}
-	return b.bless(call, email, clientName)
+	return b.bless(security.GetCall(call.Context()), email, clientName)
 }
 
-func (b *oauthBlesser) bless(call rpc.ServerCall, email, clientName string) (security.Blessings, string, error) {
+func (b *oauthBlesser) bless(call security.Call, email, clientName string) (security.Blessings, string, error) {
 	var noblessings security.Blessings
 	self := call.LocalPrincipal()
 	if self == nil {