Web Animations: Remove confusing interpolation subdirectory.

Source/core/animation/Interpolation.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef Interpolation_h
 #define Interpolation_h
 
 #include "core/animation/InterpolableValue.h"
 #include "platform/heap/Handle.h"
 
@@ skipping 27 matching lines @@
 private:
     InterpolableValue* getCachedValueForTesting() const { return m_cachedValue.get(); }
 
     friend class AnimationInterpolableValueTest;
     friend class AnimationInterpolationEffectTest;
 };
 
 }
 
 #endif