gfx: Fix sending an invalid GpuMemoryBufferHandle over mojo.

ui/gfx/mojo/struct_traits_unittest.cc

 // Copyright 2016 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 "base/message_loop/message_loop.h"
 #include "mojo/public/cpp/bindings/binding_set.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/mojo/traits_test_service.mojom.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/selection_bound.h"
@@ skipping 167 matching lines @@
   EXPECT_EQ(kModifier, output.native_pixmap_handle.planes.back().modifier);
 #endif
 
 #if !defined(OS_MACOSX) && !defined(OS_IOS)
   // TODO: Add support for mach_port on mac.
   base::SharedMemory output_memory(output.handle, true);
   EXPECT_TRUE(output_memory.Map(1024));
 #endif
 }
 
+TEST_F(StructTraitsTest, NullGpuMemoryBufferHandle) {
+  mojom::TraitsTestServicePtr proxy = GetTraitsTestProxy();
+  GpuMemoryBufferHandle output;
+  proxy->EchoGpuMemoryBufferHandle(GpuMemoryBufferHandle(), &output);
+  EXPECT_TRUE(output.is_null());
+}
+
 }  // namespace gfx