google/appengine: change managed VMs context to context.Context

This follows the change to the google.golang.org/appengine packages
to use the standard context.Context type throughout.

Change-Id: I91b456105ee29f2088c3646145ea0a940f8ffaf4
Reviewed-on: https://go-review.googlesource.com/3172
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: David Symonds <dsymonds@golang.org>

google/appengine.go

@@ -19,7 +19,7 @@ import (
 // If you are implementing a 3-legged OAuth 2.0 flow on App Engine
 // that involves user accounts, see oauth2.Config instead.
 //
-// You are required to provide a valid appengine.Context as context.
+// The provided context must have come from appengine.NewContext.
 func AppEngineTokenSource(ctx oauth2.Context, scope ...string) oauth2.TokenSource {
 	return &appEngineTokenSource{
 		ctx:         ctx,