Add MD Ink Drop to host only when a ripple/hover is active.

ui/views/animation/ink_drop_animation_observer.h

 // Copyright 2015 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 UI_VIEWS_ANIMATION_INK_DROP_ANIMATION_OBSERVER_H_
 #define UI_VIEWS_ANIMATION_INK_DROP_ANIMATION_OBSERVER_H_
 
 #include <string>
 
 #include "base/macros.h"
@@ skipping 11 matching lines @@
 
   // Notifies the observer that an animation for the given |ink_drop_state| has
   // finished and the reason for completion is given by |reason|. If |reason| is
   // SUCCESS then the animation has progressed to its final frame however if
   // |reason| is |PRE_EMPTED| then the animation was stopped before its final
   // frame.
   virtual void AnimationEnded(InkDropState ink_drop_state,
                               InkDropAnimationEndedReason reason) = 0;
 
  protected:
-  InkDropAnimationObserver() {}
-  virtual ~InkDropAnimationObserver() {}
+  InkDropAnimationObserver() = default;
+  virtual ~InkDropAnimationObserver() = default;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InkDropAnimationObserver);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_ANIMATION_INK_DROP_ANIMATION_OBSERVER_H_