Mojo C++ bindings: fix inlined union validation.

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

 {%- import "validation_macros.tmpl" as validation_macros %}
 {%- set class_name = union.name ~ "_Data" %}
 {%- set enum_name = union.name ~ "_Tag" -%}
 
 // static
 bool {{class_name}}::Validate(
     const void* data,
     mojo::internal::ValidationContext* validation_context,
     bool inlined) {
-  if (!data)
+  if (!data) {
+    DCHECK(!inlined);
     return true;
+  }
 
-  if (!ValidateUnionHeaderAndClaimMemory(data, inlined, validation_context))
+  // If it is inlined, the alignment is already enforced by its enclosing
+  // object. We don't have to validate that.
+  DCHECK(!inlined || mojo::internal::IsAligned(data));
+
+  if (!inlined &&
+      !mojo::internal::ValidateNonInlinedUnionHeaderAndClaimMemory(
+          data, validation_context)) {
     return false;
+  }
 
   const {{class_name}}* object = static_cast<const {{class_name}}*>(data);
   ALLOW_UNUSED_LOCAL(object);
 
+  if (inlined && object->is_null())
+    return true;
+
   switch (object->tag) {
 {%  for field in union.fields %}
     case {{enum_name}}::{{field.name|upper}}: {
 {%-   set field_expr = "object->data.f_" ~ field.name %}
 {{validation_macros.validate_field(field, field_expr, union.name, false)|indent(4)}}
       return true;
     }
 {%- endfor %}
     default: {
       ReportValidationError(
           validation_context,
           mojo::internal::VALIDATION_ERROR_UNKNOWN_UNION_TAG,
           "unknown tag in {{union.name}}");
       return false;
     }
   }
 }