impl/prod: Embed AppEngine SDK into Context.

impl/prod/context_vm.go

Index: impl/prod/context_vm.go
diff --git a/impl/prod/context_vm.go b/impl/prod/context_vm.go
index 874fc90fcefcf148d5a4e88377af2d7f35829659..cffdde7dc9ce0c2036c9b8032b39754428bde6d7 100644
--- a/impl/prod/context_vm.go
+++ b/impl/prod/context_vm.go
@@ -15,6 +15,10 @@ import (
 // implementations which aren't associated with any particular request.
 //
 // This is only available on Managed VMs.
-func UseBackground(c context.Context) context.Context {
-	return setupAECtx(c, appengine.BackgroundContext())
+//
+// In addition, UseBackground installs Google AppEngine SDK Context values into
+// the Context. See the warning in Use for more information on pitfalls related
+// to usage of the raw AppEngine SDKs alongside luci/gae.
+func UseBackground() context.Context {
+	return setupAECtx(appengine.BackgroundContext())
 }