Document why ComputedStyle constructor and destructor are protected.

third_party/WebKit/Source/build/scripts/templates/ComputedStyleBase.h.tmpl

 {% from 'macros.tmpl' import license, print_if %}
 {{license()}}
 
 #ifndef ComputedStyleBase_h
 #define ComputedStyleBase_h
 
 #include "core/style/ComputedStyleConstants.h"
 #include "core/CoreExport.h"
 {% for path in include_paths %}
 #include "{{path}}"
@@ skipping 61 matching lines @@
   // Fields.
   // TODO(sashab): Remove initialFoo() static methods and update callers to
   // use resetFoo(), which can be more efficient.
 
   {% for field in fields %}
   // {{field.property_name}}
   {{field_templates[field.field_template].decl_methods(field)|indent(2)}}
 
   {% endfor %}
  protected:
+  // Constructor and destructor are protected so that only the parent class ComputedStyle
+  // can instantiate this class.
   ALWAYS_INLINE ComputedStyleBase() :
   {% for field in fields %}
       {{field.name}}(static_cast<unsigned>({{field.default_value}})){{print_if(not loop.last, ',')}}
   {% endfor %}
   {}
 
   ~ComputedStyleBase() = default;
 
   // Storage.
   {% for field in fields %}
   unsigned {{field.name}} : {{field.size}}; // {{field.type_name}}
   {% endfor %}
 };
 
 } // namespace blink
 
 #endif // ComputedStyleBase_h