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

ui/views/animation/ink_drop_hover_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_HOVER_OBSERVER_H_
+#define UI_VIEWS_ANIMATION_INK_DROP_HOVER_OBSERVER_H_
+
+#include <string>
+
+#include "base/macros.h"
+#include "ui/views/animation/ink_drop_animation_ended_reason.h"
+#include "ui/views/animation/ink_drop_hover.h"
+#include "ui/views/views_export.h"
+
+namespace views {
+
+// Observer to attach to an InkDropHover animation.
+class VIEWS_EXPORT InkDropHoverObserver {
+ public:
+  // An animation for the given |animation_type| has started.
+  virtual void AnimationStarted(InkDropHover::AnimationType animation_type) = 0;
+
+  // Notifies the observer that an animation for the given |animation_type| 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(InkDropHover::AnimationType animation_type,
+                              InkDropAnimationEndedReason reason) = 0;
+
+ protected:
+  InkDropHoverObserver() {}

[varkha, 2016/04/28 20:35:00]

= default here and for dtor.

[bruthig, 2016/04/29 11:02:35]

Done.

+  virtual ~InkDropHoverObserver() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(InkDropHoverObserver);
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_ANIMATION_INK_DROP_HOVER_OBSERVER_H_