Makes ViewManagerServiceImpl track a single root

mojo/services/view_manager/access_policy_delegate.h

Index: mojo/services/view_manager/access_policy_delegate.h
diff --git a/mojo/services/view_manager/access_policy_delegate.h b/mojo/services/view_manager/access_policy_delegate.h
index 12c7479387c8df074cda20545d62277879b7b1e9..e32c79dee70416a35c12fe904f6585e2778d3de6 100644
--- a/mojo/services/view_manager/access_policy_delegate.h
+++ b/mojo/services/view_manager/access_policy_delegate.h
@@ -18,9 +18,8 @@ class ServerView;
 // Delegate used by the AccessPolicy implementations to get state.
 class AccessPolicyDelegate {
  public:
-  // Returns the ids of the roots views for this connection. That is, this is
-  // the set of views the connection was embedded at.
-  virtual const base::hash_set<Id>& GetRootsForAccessPolicy() const = 0;
+  // Returns true if |id| is the root of the connection.
+  virtual bool IsRootForAccessPolicy(const ViewId& id) const = 0;
 
   // Returns true if |view| has been exposed to the client.
   virtual bool IsViewKnownForAccessPolicy(const ServerView* view) const = 0;