tab_hover_card: set anchor_view_ to null when stopping animation

This isn't necessarily a problem right now, but without this it's
entirely possible that WidgetSlideAnimationDelegate is holding a
reference to a deleted view, which makes me nervous.

I came across this when looking at a crash that was the result of a UAF
that I suspect was in this code. I believe you've since fixed it.

BUG=940322
TEST=none

Change-Id: Icfacdb7533ab9d1e8b2d99cbb29b459e2a21445a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627654Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662754}

chrome/browser/ui/views/tabs/tab_hover_card_bubble_view.cc

@@ -232,7 +232,10 @@ class TabHoverCardBubbleView::WidgetSlideAnimationDelegate
     slide_animation_->Show();
   }
 
-  void StopAnimation() { slide_animation_->Stop(); }
+  void StopAnimation() {
+    anchor_view_ = nullptr;
+    slide_animation_->Stop();
+  }
 
   bool IsAnimating() { return slide_animation_->is_animating(); }
 
@@ -251,10 +254,12 @@ class TabHoverCardBubbleView::WidgetSlideAnimationDelegate
 
   TabHoverCardBubbleView* const bubble_delegate_;
   std::unique_ptr<gfx::SlideAnimation> slide_animation_;
-  views::View* anchor_view_;
+  views::View* anchor_view_ = nullptr;
   gfx::Rect starting_bubble_bounds_;
   gfx::Rect target_bubble_bounds_;
   gfx::Rect current_bubble_bounds_;
+
+  DISALLOW_COPY_AND_ASSIGN(WidgetSlideAnimationDelegate);
 };
 
 TabHoverCardBubbleView::TabHoverCardBubbleView(Tab* tab)