Do not play assistant end animation when hiding

When assistant is started and triggered hiding animation, during a very
short time window if the user lifted their finger, we would try to play
end animation.  We should avoid doing that.

BUG=b:67496461
TEST=build and flash locally, see animation playing normally

Change-Id: Idf52f692edd9d05b16ad48687384d291b208a42e
Reviewed-on: https://chromium-review.googlesource.com/748268Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513208}

ash/shelf/voice_interaction_overlay.cc

@@ -851,8 +851,9 @@ void VoiceInteractionOverlay::WaitingAnimation() {
 }
 
 void VoiceInteractionOverlay::EndAnimation() {
-  if (IsBursting()) {
+  if (IsBursting() || IsHidden()) {
     // Too late, user action already fired, we have to finish what's started.
+    // Or the widget has already been hidden, no need to play the end animation.
     return;
   }
 

ash/shelf/voice_interaction_overlay.h

@@ -33,6 +33,7 @@ class ASH_EXPORT VoiceInteractionOverlay : public views::View {
     return AnimationState::BURSTING == animation_state_;
   }
   bool IsWaiting() const { return AnimationState::WAITING == animation_state_; }
+  bool IsHidden() const { return AnimationState::HIDDEN == animation_state_; }
 
  private:
   enum class AnimationState {