Bug 1641245 - Make string comparators not virtual. r=froydnj,necko-reviewers,geckoview-reviewers,jgilbert,agi,valentin

There's no use case for stateful comparators, so they can be just plain
function pointers.

This is used in some hot places like CSS selector matching.

Differential Revision: https://phabricator.services.mozilla.com/D77084

widget/windows/KeyboardLayout.cpp

@@ -1010,8 +1010,7 @@ bool UniCharsAndModifiers::UniCharsEqual(
 
 bool UniCharsAndModifiers::UniCharsCaseInsensitiveEqual(
     const UniCharsAndModifiers& aOther) const {
-  nsCaseInsensitiveStringComparator comp;
-  return mChars.Equals(aOther.mChars, comp);
+  return mChars.Equals(aOther.mChars, nsCaseInsensitiveStringComparator);
 }
 
 bool UniCharsAndModifiers::BeginsWith(