Mojo C++ bindings: provide data view for all object types.

mojo/public/tools/bindings/generators/cpp_templates/union_data_view_declaration.tmpl

Index: mojo/public/tools/bindings/generators/cpp_templates/union_data_view_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/union_data_view_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/union_data_view_declaration.tmpl
new file mode 100644
index 0000000000000000000000000000000000000000..d9ab16a7fef6b72526ad8019d2ee6a1c32f7f1fe
--- /dev/null
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_data_view_declaration.tmpl
@@ -0,0 +1,17 @@
+class {{union.name}}DataView {
+ public:
+  {{union.name}}DataView() {}
+  
+  {{union.name}}DataView(
+      internal::{{union.name}}_Data* data,
+      mojo::internal::SerializationContext* context)
+      : data_(data) {}
+
+  bool is_null() const { return !data_; }
+ 
+  // TODO(yzshen): Union data view is not yet supported.
+
+ private:
+  internal::{{union.name}}_Data* data_ = nullptr;
+};
+