overview: Speculative fix for clusterfuzz issue.

Could not repro the issue but based on the stack trace, looks like post
animation code is running after window is destroyed, so stop observing
animations when object is destroyed.

Test: none
Bug: 899459
Change-Id: Ia1ec07057281c91aaa390ab90df28910fa74aaac
Reviewed-on: https://chromium-review.googlesource.com/c/1308610Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604372}

ash/wm/overview/scoped_transform_overview_window.cc

@@ -186,7 +186,9 @@ ScopedTransformOverviewWindow::ScopedTransformOverviewWindow(
   type_ = GetWindowDimensionsType(window);
 }
 
-ScopedTransformOverviewWindow::~ScopedTransformOverviewWindow() = default;
+ScopedTransformOverviewWindow::~ScopedTransformOverviewWindow() {
+  StopObservingImplicitAnimations();
+}
 
 // static
 float ScopedTransformOverviewWindow::GetItemScale(const gfx::Size& source,