mojo: Make InterfacePtr<> testable in if() statements without .get().

mojo/public/cpp/bindings/interface_ptr.h

Index: mojo/public/cpp/bindings/interface_ptr.h
diff --git a/mojo/public/cpp/bindings/interface_ptr.h b/mojo/public/cpp/bindings/interface_ptr.h
index b868a0a5c36167c1885c16f597a600bfed4ecef2..01b15ed047d579df025a435d56433612ff5c49fa 100644
--- a/mojo/public/cpp/bindings/interface_ptr.h
+++ b/mojo/public/cpp/bindings/interface_ptr.h
@@ -104,6 +104,16 @@ class InterfacePtr {
     return &internal_state_;
   }
 
+  // Allow InterfacePtr<> to be used in boolean expressions, but not
+  // implicitly convertible to a real bool (which is dangerous).
+ private:
+  typedef internal::InterfacePtrState<Interface> InterfacePtr::*Testable;
+
+ public:
+  operator Testable() const {
+    return internal_state_.is_bound() ? &InterfacePtr::internal_state_ : NULL;
+  }
+
  private:
   typedef internal::InterfacePtrState<Interface> State;
   mutable State internal_state_;