Avoid repeating the list of CSS image functions

lib/sanitize/css.rb

@@ -229,6 +229,12 @@ def at_rule!(rule)
  rule
  end
 
+ # Returns `true` if the given CSS function name is an image-related function
+ # that may contain image URLs that need to be validated.
+ def image_function?(name)
+ ['image', 'image-set', '-webkit-image-set'].include?(name)
+ end
+
  # Passes the URL value of an @import rule to a block to ensure
  # it's an allowed URL
  def import_url_allowed?(rule)
@@ -272,7 +278,7 @@ def property!(prop)
  return nil unless valid_url?(child)
  end
 
- if ['image-set', 'image', '-webkit-image-set'].include?(name)
+ if image_function?(name)
  return nil unless valid_image?(child)
  end
 
@@ -349,11 +355,11 @@ def valid_url?(node)
  false
  end
 
- # Returns `true` if the given node (which is an `image` or `image-set` function) contains only strings
- # using an allowlisted protocol.
+ # Returns `true` if the given node is an image-related function and contains
+ # only strings that use an allowlisted protocol.
  def valid_image?(node)
  return false unless node[:node] == :function
- return false unless node.key?(:name) && ['image', 'image-set', '-webkit-image-set'].include?(node[:name].downcase)
+ return false unless node.key?(:name) && image_function?(node[:name].downcase)
  return false unless Array === node[:value]
 
  node[:value].each do |token|