Web Animations: Remove legacy animations engine.

Source/core/animation/css/CSSAnimations.cpp

Index: Source/core/animation/css/CSSAnimations.cpp
diff --git a/Source/core/animation/css/CSSAnimations.cpp b/Source/core/animation/css/CSSAnimations.cpp
index 897f5dd299d75bf59b5a85779c196be42e595df9..a6f22b783cde009273275586cefc77a2e81311f1 100644
--- a/Source/core/animation/css/CSSAnimations.cpp
+++ b/Source/core/animation/css/CSSAnimations.cpp
@@ -900,11 +900,6 @@ bool CSSAnimations::isAnimatableProperty(CSSPropertyID property)
     case CSSPropertyZIndex:
     case CSSPropertyZoom:
         return true;
-    // FIXME: Shorthands should not be present in this list, but
-    // CSSPropertyAnimation implements animation of these shorthands
-    // directly and makes use of this method.
-    case CSSPropertyFlex:
-        return !RuntimeEnabledFeatures::webAnimationsCSSEnabled();
     default:
         return false;
     }