Allow constructing {Interface,Struct}Ptr from nullptr

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

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/public/cpp/bindings/binding.h"
 #include "mojo/public/cpp/bindings/error_handler.h"
 #include "mojo/public/cpp/bindings/strong_binding.h"
 #include "mojo/public/cpp/environment/environment.h"
 #include "mojo/public/cpp/utility/run_loop.h"
 #include "mojo/public/interfaces/bindings/tests/math_calculator.mojom.h"
@@ skipping 309 matching lines @@
 
   PumpMessages();
 
   EXPECT_EQ(0, SelfDestructingMathCalculatorUIImpl::num_instances());
 }
 
 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();
 
   EXPECT_EQ(2, impl->max_call_depth());
 }
 
 class StrongMathCalculatorImpl : public math::Calculator, public ErrorHandler {
  public:
   StrongMathCalculatorImpl(ScopedMessagePipeHandle handle,
                            bool* error_received,
@@ skipping 131 matching lines @@
   }
 
   loop.RunUntilIdle();
   EXPECT_TRUE(error_received);
   EXPECT_FALSE(destroyed);
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace mojo