Refactored out need to pass property ID in background list parsing.

third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h

Index: third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h
diff --git a/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h b/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h
index 08bb57cfc8a5ae117a798c04b12a9fcc80f61d5e..bc2003010609167822e07e270f47ab4ae11bd9ae 100644
--- a/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h
+++ b/third_party/WebKit/Source/core/css/parser/CSSPropertyParserHelpers.h
@@ -112,6 +112,8 @@ bool IsCSSWideKeyword(StringView);
 
 CSSIdentifierValue* ConsumeShapeBox(CSSParserTokenRange&);
 
+void AddListValueOptimized(CSSValue*& list, CSSValue*);
+
 // Template implementations are at the bottom of the file for readability.
 
 template <typename... emptyBaseCase>
@@ -131,22 +133,33 @@ CSSIdentifierValue* ConsumeIdent(CSSParserTokenRange& range) {
   return CSSIdentifierValue::Create(range.ConsumeIncludingWhitespace().Id());
 }
 
-// ConsumeCommaSeparatedList takes a callback function to call on each item in
+enum class Optimise { kNo, kYes };
+
+// The first argument to ConsumeCommaSeparatedList indicates whether to optimise
+// the list for memory, such that a single value is returned as it is, instead
+// of being wrapped in a list. Using the optimisation may worsen code health

[suzyh_UTC10 (ex-contributor), 2017/06/05 00:49:20]

Do we have data to indicate that the optimisation does in fact have a
performance benefit?

[Bugs Nash, 2017/06/05 01:20:50]

currently working on that. it seems like it does have a performance benefit but
further investigation required. happy to hold off on this patch until that
happens

+// as the result needs to be handled for both list and single value cases.
+// The following arguments are a callback function to call on each item in
 // the list, followed by the arguments to pass to this callback.
 // The first argument to the callback must be the CSSParserTokenRange
 template <typename Func, typename... Args>
-CSSValueList* ConsumeCommaSeparatedList(Func callback,
-                                        CSSParserTokenRange& range,
-                                        Args... args) {
-  CSSValueList* list = CSSValueList::CreateCommaSeparated();
+CSSValue* ConsumeCommaSeparatedList(Optimise optimise,
+                                    Func callback,
+                                    CSSParserTokenRange& range,
+                                    Args... args) {
+  DCHECK(optimise == Optimise::kNo || optimise == Optimise::kYes);
+  CSSValue* result = nullptr;
+  if (optimise == Optimise::kNo)
+    result = CSSValueList::CreateCommaSeparated();
   do {
     CSSValue* value = callback(range, args...);
     if (!value)
       return nullptr;
-    list->Append(*value);
+    optimise == Optimise::kYes ? AddListValueOptimized(result, value)
+                               : ToCSSValueList(result)->Append(*value);
   } while (ConsumeCommaIncludingWhitespace(range));
-  DCHECK(list->length());
-  return list;
+  DCHECK(!result->IsBaseValueList() || ToCSSValueList(result)->length());
+  return result;
 }
 
 }  // namespace CSSPropertyParserHelpers