Add a comma delimited syntax parser and use it for Accept-CH

Source/core/fetch/Resource.cpp

Index: Source/core/fetch/Resource.cpp
diff --git a/Source/core/fetch/Resource.cpp b/Source/core/fetch/Resource.cpp
index e7025397a44cebfef585efe994ab88260702c160..d7628199c448966a8d98ebe460c45cc896dea394 100644
--- a/Source/core/fetch/Resource.cpp
+++ b/Source/core/fetch/Resource.cpp
@@ -445,8 +445,8 @@ void Resource::responseReceived(const ResourceResponse& response, PassOwnPtr<Web
         if (fetcher && fetcher->frame()) {
             LinkLoader::loadLinkFromHeader(response.httpHeaderField("Link"), fetcher->frame()->document());
             if (RuntimeEnabledFeatures::clientHintsEnabled() && type() == Resource::MainResource) {
-                String acceptCH = response.httpHeaderField("accept-ch").lower();
-                // FIXME: Write an actual parser for this comma delimited header. crbug.com/461741
+                CommaDelimitedHeaderSet acceptCH;
+                parseCommaDelimitedHeader(response.httpHeaderField("accept-ch"), acceptCH);
                 if (acceptCH.contains("dpr"))
                     fetcher->frame()->setShouldSendDPRHint();
                 if (acceptCH.contains("rw"))