Deprecate step timing with position 'middle'

third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h

Index: third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h
diff --git a/third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h b/third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h
index 27b10ae6fc5b74a37c372962253ea4c31d70e83a..cfa9f969c18ca967cc0a95cd1ea9eaebb825827c 100644
--- a/third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h
+++ b/third_party/WebKit/Source/core/css/resolver/CSSToStyleMap.h
@@ -32,6 +32,7 @@
 
 namespace blink {
 
+class Document;
 class FillLayer;
 class CSSValue;
 class StyleResolverState;
@@ -76,9 +77,13 @@ class CSSToStyleMap {
   static EAnimPlayState MapAnimationPlayState(const CSSValue&);
   static CSSTransitionData::TransitionProperty MapAnimationProperty(
       const CSSValue&);
+
+  // Pass a Document* if allow_step_middle is true so that the usage can be
+  // counted.
   static PassRefPtr<TimingFunction> MapAnimationTimingFunction(
       const CSSValue&,
-      bool allow_step_middle = false);
+      bool allow_step_middle = false,
+      Document* = nullptr);
 
   static void MapNinePieceImage(StyleResolverState&,
                                 CSSPropertyID,