Mojo C++ bindings: share DataView class between chromium and blink variants.

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

+{%- set mojom_type = union|get_qualified_name_for_kind %}
+
+template <>
+struct {{export_attribute}} UnionTraits<{{mojom_type}}::DataView,
+                                        {{mojom_type}}Ptr> {
+  static bool IsNull(const {{mojom_type}}Ptr& input) { return !input; }
+  static void SetToNull({{mojom_type}}Ptr* output) { output->reset(); }
+
+  static {{mojom_type}}::Tag GetTag(const {{mojom_type}}Ptr& input) {
+    return input->which();
+  }
+
+{%- for field in union.fields %}
+{%-   set return_ref = field.kind|is_object_kind or
+                       field.kind|is_any_handle_or_interface_kind %}
+{%-   if return_ref %}
+  static decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())
+      {{field.name}}({{mojom_type}}Ptr& input) {
+    return input->get_{{field.name}}();
+  }
+{%-   else %}
+  static decltype(std::declval<{{mojom_type}}>().get_{{field.name}}())
+      {{field.name}}(const {{mojom_type}}Ptr& input) {
+    return input->get_{{field.name}}();
+  }
+{%-   endif %}
+{%- endfor %}
+
+  static bool Read({{mojom_type}}::DataView input, {{mojom_type}}Ptr* output);
+};