Allow constructing {Interface,Struct}Ptr from nullptr

mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc

Index: mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc b/mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc
index 91ce84b7d9030865555b1ab12583b242bc810e58..f90dc3de1ea13e82398f0df4283aecab9834efff 100644
--- a/mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/interface_ptr_unittest.cc
@@ -327,12 +327,8 @@ TEST_F(InterfacePtrTest, ReentrantWaitForIncomingMethodCall) {
   sample::ServicePtr proxy;
   ReentrantServiceImpl* impl = BindToProxy(new ReentrantServiceImpl(), &proxy);
 
-  proxy->Frobinate(sample::FooPtr(),
-                   sample::Service::BAZ_OPTIONS_REGULAR,
-                   sample::PortPtr());
-  proxy->Frobinate(sample::FooPtr(),
-                   sample::Service::BAZ_OPTIONS_REGULAR,
-                   sample::PortPtr());
+  proxy->Frobinate(nullptr, sample::Service::BAZ_OPTIONS_REGULAR, nullptr);

[jamesr, 2015/01/13 23:27:05]

this is the example use. The first parameter to this function is a StructPtr,
the third is an InterfacePtr.

+  proxy->Frobinate(nullptr, sample::Service::BAZ_OPTIONS_REGULAR, nullptr);
 
   PumpMessages();