overview: Speculative fix for ash::WindowSelector::OnGridEmpty.

The crash stack shows OnGridEmpty being called because of WindowDestroyed
while overview is shutting down because of leaving tablet mode.

The feedbacks report say this is play store related, but i have been
unable to reproduce.

This cl introduces a early exit in OnGridEmpty if overview is currently
shutting down. The cause seems like play store is getting destroyed while
in overview tablet and perhaps causing a race condition, but we should
prevent overview from crashing if play store closes its window
unexpectedly.

Test: none
Bug: 881089
Change-Id: I8e8daceb53d2ac56f92662b14d33fd86979dff25
Reviewed-on: https://chromium-review.googlesource.com/1256168Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595734}

ash/wm/overview/window_selector.cc

@@ -463,6 +463,11 @@ void WindowSelector::CancelSelection() {
 }
 
 void WindowSelector::OnGridEmpty(WindowGrid* grid) {
+  // TODO(crbug.com/881089): Speculative fix based on the crash stack, needs
+  // confirming.
+  if (IsShuttingDown())
+    return;
+
   size_t index = 0;
   // If there are no longer any items on any of the grids, shutdown,
   // otherwise the empty grids will remain blurred but will have no items.