Add support for generating external types in ComputedStyleBase.

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

Index: third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
diff --git a/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl b/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
new file mode 100644
index 0000000000000000000000000000000000000000..8790c220a74458dfa70c7a5238d35b8e99f6a1ab
--- /dev/null
+++ b/third_party/WebKit/Source/build/scripts/templates/fields/field.tmpl
@@ -0,0 +1,23 @@
+{% macro encode(field, value) %}
+{% if field.is_bit_field -%}
+static_cast<unsigned>({{value}})
+{%- else -%}
+{{value}}
+{%- endif %}
+{% endmacro %}
+
+{% macro decode(field, value) %}
+{% if field.is_bit_field -%}
+static_cast<{{field.type_name}}>({{value}})
+{%- else -%}
+{{value}}
+{%- endif %}
+{% endmacro %}
+
+{% macro return_type(field) %}
+{% if field.is_bit_field -%}
+{{field.type_name}}
+{%- else -%}
+const {{field.type_name}}&
+{%- endif %}
+{% endmacro %}