Web Animations API: Implement step-middle and steps(x, middle) timing functions.

Source/core/css/parser/BisonCSSParser.h

Index: Source/core/css/parser/BisonCSSParser.h
diff --git a/Source/core/css/parser/BisonCSSParser.h b/Source/core/css/parser/BisonCSSParser.h
index a0ce239ea848dcd5cd0402edd1e6fc519b4a5369..2a79d0d1137ae8e0b0052ba52254863ed98827bd 100644
--- a/Source/core/css/parser/BisonCSSParser.h
+++ b/Source/core/css/parser/BisonCSSParser.h
@@ -88,11 +88,11 @@ public:
     PassRefPtr<StyleRuleBase> parseRule(StyleSheetContents*, const String&);
     PassRefPtr<StyleKeyframe> parseKeyframeRule(StyleSheetContents*, const String&);
     bool parseSupportsCondition(const String&);
-    static bool parseValue(MutableStylePropertySet*, CSSPropertyID, const String&, bool important, CSSParserMode, StyleSheetContents*);
+    static bool parseValue(MutableStylePropertySet*, CSSPropertyID, const String&, bool important, CSSParserMode, StyleSheetContents*, bool parseStepMiddleTimingFunction = false);
     static bool parseColor(RGBA32& color, const String&, bool strict = false);
     static bool parseSystemColor(RGBA32& color, const String&, Document*);
     static PassRefPtr<CSSValueList> parseFontFaceValue(const AtomicString&);
-    static PassRefPtr<CSSValue> parseAnimationTimingFunctionValue(const String&);
+    static PassRefPtr<CSSValue> parseAnimationTimingFunctionValue(const String&, bool parseStepMiddleTimingFunction = false);
     PassRefPtr<CSSPrimitiveValue> parseValidPrimitive(CSSValueID ident, CSSParserValue*);
     bool parseDeclaration(MutableStylePropertySet*, const String&, CSSParserObserver*, StyleSheetContents* contextStyleSheet);
     static PassRefPtr<ImmutableStylePropertySet> parseInlineStyleDeclaration(const String&, Element*);
@@ -383,6 +383,7 @@ public:
     bool m_hadSyntacticallyValidCSSRule;
     bool m_logErrors;
     bool m_ignoreErrors;
+    bool m_parseStepMiddleTimingFunction;
 
     AtomicString m_defaultNamespace;