Replace emoji with mathematical symbol in CSS selector ref test

Replace the snowman emoji (U+2603) with the sum sign (U+2211) in order
to avoid situations in which a color fallback font is chosen for the
emoji, which cannot be colored white and the reftest thus failing, since
the test case shows and emoji, while the reference content does not
contain any characters.

Bug: 976737
Change-Id: I342d81870418e56575700d3c0941a16544121a57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760304
Auto-Submit: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688076}

third_party/blink/web_tests/external/wpt/css/selectors/selectors-empty-001.xml

@@ -35,7 +35,7 @@
     <test1><test1> </test1></test1>
     <test2>green!</test2>
     <test3>&gt;&lt;</test3>
-    <test4>&#x2603;</test4>
+    <test4>&#x2211;</test4>
     <test5><test5/></test5>
     <test6> </test6>
     <test7> <!-- --></test7>