Make PropertyHandle instances const references where possible

third_party/WebKit/Source/core/animation/Keyframe.cpp

 // Copyright 2016 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.
 
 #include "core/animation/Keyframe.h"
 
 #include "core/animation/InvalidatableInterpolation.h"
 
 namespace blink {
 
 PassRefPtr<Interpolation>
 Keyframe::PropertySpecificKeyframe::createInterpolation(
-    PropertyHandle propertyHandle,
+    const PropertyHandle& propertyHandle,
     const Keyframe::PropertySpecificKeyframe& end) const {
   // const_cast to take refs.
   return InvalidatableInterpolation::create(
       propertyHandle, const_cast<PropertySpecificKeyframe*>(this),
       const_cast<PropertySpecificKeyframe*>(&end));
 }
 
 }  // namespace blink