Fetch, XHR: Use ", " as header list value separator.

third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp

Index: third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp
diff --git a/third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp b/third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp
index c67d74153a48db40b9ef3378c81efdc9db0ac536..60db84572149354eff3a0ab4487921945843322e 100644
--- a/third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp
+++ b/third_party/WebKit/Source/platform/loader/fetch/ResourceRequest.cpp
@@ -328,7 +328,7 @@ void ResourceRequest::AddHTTPHeaderField(const AtomicString& name,
                                          const AtomicString& value) {
   HTTPHeaderMap::AddResult result = http_header_fields_.Add(name, value);
   if (!result.is_new_entry)
-    result.stored_value->value = result.stored_value->value + ',' + value;
+    result.stored_value->value = result.stored_value->value + ", " + value;
 }
 
 void ResourceRequest::AddHTTPHeaderFields(const HTTPHeaderMap& header_fields) {