gfx: Fix sending an invalid GpuMemoryBufferHandle over mojo.

ui/gfx/mojo/struct_traits_unittest.cc

Index: ui/gfx/mojo/struct_traits_unittest.cc
diff --git a/ui/gfx/mojo/struct_traits_unittest.cc b/ui/gfx/mojo/struct_traits_unittest.cc
index d837cd82da0c3b436638acd228994b71a9fa2eb1..e7cb15f72250c6cd519b9b19ea3e799d56d932b2 100644
--- a/ui/gfx/mojo/struct_traits_unittest.cc
+++ b/ui/gfx/mojo/struct_traits_unittest.cc
@@ -185,4 +185,11 @@ TEST_F(StructTraitsTest, GpuMemoryBufferHandle) {
 #endif
 }
 
+TEST_F(StructTraitsTest, NullGpuMemoryBufferHandle) {
+  mojom::TraitsTestServicePtr proxy = GetTraitsTestProxy();
+  GpuMemoryBufferHandle output;
+  proxy->EchoGpuMemoryBufferHandle(GpuMemoryBufferHandle(), &output);
+  EXPECT_TRUE(output.is_null());
+}
+
 }  // namespace gfx