Fix ComputedStyleBase.h.tmpl to generate accessors for fields on groups

third_party/WebKit/Source/build/scripts/templates/fields/base.tmpl

 {% from 'fields/field.tmpl' import encode, decode, const_ref, nonconst_ref, getter_expression, setter_expression %}
 
 {% macro decl_initial_method(field) -%}
 inline static {{field.type_name}} {{field.initial_method_name}}() {
   return {{field.default_value}};
 }
 {%- endmacro %}
 
 {% macro decl_getter_method(field) -%}
 {{const_ref(field)}} {{field.getter_method_name}}() const {
   return {{decode(field, getter_expression(field))}};
 }
 {%- endmacro %}
 
 {% macro decl_setter_method(field) -%}
 void {{field.setter_method_name}}({{const_ref(field)}} v) {
   {{setter_expression(field)}} = {{encode(field, "v")}};
 }
 {%- endmacro %}
 
 {% macro decl_resetter_method(field) -%}
 inline void {{field.resetter_method_name}}() {
   {{setter_expression(field)}} = {{encode(field, field.default_value)}};
 }
 {%- endmacro %}
 
 {% macro decl_mutable_method(field) -%}
-{{nonconst_ref(field)}} {{field.mutable_method_name}}() const {
-  return {{decode(field, getter_expression(field))}};
+{{nonconst_ref(field)}} {{field.internal_mutable_method_name}}() {
+  return {{decode(field, setter_expression(field))}};
 }
 {%- endmacro %}
 
 {% macro decl_internal_getter_method(field) -%}
 {{const_ref(field)}} {{field.internal_getter_method_name}}() const {
   return {{decode(field, getter_expression(field))}};
 }
 {%- endmacro %}
 
 {% macro decl_internal_setter_method(field) -%}
 void {{field.internal_setter_method_name}}({{const_ref(field)}} v) {
   {{setter_expression(field)}} = {{encode(field, "v")}};
 }
 {%- endmacro %}