Web Animations API: Deferred computation of interpolated values

Source/core/css/resolver/StyleResolver.cpp

Index: Source/core/css/resolver/StyleResolver.cpp
diff --git a/Source/core/css/resolver/StyleResolver.cpp b/Source/core/css/resolver/StyleResolver.cpp
index 5c412c674ed019f230ad475fbccdcba2dba638f4..3e16b5b75405607b697a890f0698808de0b86749 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -755,14 +755,19 @@ PassRefPtr<RenderStyle> StyleResolver::styleForKeyframe(Element* element, const
 
 // This function is used by the WebAnimations JavaScript API method animate().
 // FIXME: Remove this when animate() switches away from resolution-dependent parsing.
-PassRefPtrWillBeRawPtr<AnimatableValue> StyleResolver::createAnimatableValueSnapshot(Element& element, CSSPropertyID property, CSSValue* value)
+PassRefPtrWillBeRawPtr<AnimatableValue> StyleResolver::applyAndSnapshotAnimatableValue(Element& element, CSSPropertyID property, CSSValue& value, RenderStyle* mutableStyle)
 {
-    // We use a fresh RenderStyle here because certain values (eg. background-position) won't always completely replace the previously applied property.
-    RefPtr<RenderStyle> style = element.renderStyle() ? RenderStyle::clone(element.renderStyle()) : RenderStyle::create();
+    RefPtr<RenderStyle> style;
+    if (mutableStyle)
+        style = mutableStyle;
+    else if (element.renderStyle())
+        style = RenderStyle::clone(element.renderStyle());
+    else
+        style = RenderStyle::create();
     StyleResolverState state(element.document(), &element);
     state.setStyle(style.get());
     state.fontBuilder().initForStyleResolve(state.document(), state.style(), state.useSVGZoomRules());
-    StyleBuilder::applyProperty(property, state, value);
+    StyleBuilder::applyProperty(property, state, &value);
     return CSSAnimatableValueFactory::create(property, *style);
 }