[Mojo] Eliminate leak in InterfacePtrSet

InterfacePtrSet relies on its ELements to delete themselves. In turn
they rely on receiving a connection error callback from their
InterfacePtr. However, when InterfacePtrSet is destroyed it calls
Element::Close() on any live Elements. Element::Close() resets the
InterfacePtr, meaning that that Entry will *not* receive a connection
error callback from that InterfacePtr at any point in the future and
so will leak.

This CL eliminates the leak by having Element::Close() delete the
Element.

Review-Url: https://codereview.chromium.org/2471033002
Cr-Commit-Position: refs/heads/master@{#429168}

mojo/public/cpp/bindings/interface_ptr_set.h

@@ -16,6 +16,10 @@
 namespace mojo {
 namespace internal {
 
+// TODO(blundell): This class should be rewritten to be structured
+// similarly to BindingSet if possible, with PtrSet owning its
+// Elements and those Elements calling back into PtrSet on connection
+// error.
 template <typename Interface, template <typename> class Ptr>
 class PtrSet {
  public:
@@ -55,7 +59,13 @@ class PtrSet {
 
     ~Element() {}
 
-    void Close() { ptr_.reset(); }
+    void Close() {
+      ptr_.reset();
+
+      // Resetting the interface ptr means that it won't call this object back
+      // on connection error anymore, so this object must delete itself now.
+      DeleteElement(this);
+    }
 
     Interface* get() { return ptr_.get(); }