Escape characters when serializing CSS ::highlight() pseudo-element's argument.

Bug: 495819158
Change-Id: Ia027a070e60dda02132cf0d7baea1253f6d76b12

Author: dbaron

css/css-highlight-api/highlight-pseudo-parsing.html

@@ -12,6 +12,9 @@
   test_valid_selector(`div ${pseudo}`);
   test_valid_selector(`::part(my-part)${pseudo}`);
 
+  test_valid_selector("::highlight(multi\\ word)");
+  test_valid_selector("::highlight(\\31\\32\\33)", "::highlight(\\31 23)");
+
   test_invalid_selector("::highlight");
   test_invalid_selector(`::before${pseudo}`);
   test_invalid_selector(`${pseudo}.a`);