Desks: Fix a crash when ChromeVox is enabled

When a mini_view is removed, it outlives its desk, but ChromeVox
may try to get the accessible node data at this point when `desk_`
is nullptr.

BUG=999595
TEST=Manual

Change-Id: I62ebc62f4dee67e03da6fb5d588520451f9e85ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779184Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692142}

ash/wm/desks/desk_mini_view.cc

@@ -186,7 +186,8 @@ gfx::Size DeskMiniView::CalculatePreferredSize() const {
 void DeskMiniView::GetAccessibleNodeData(ui::AXNodeData* node_data) {
   views::Button::GetAccessibleNodeData(node_data);
 
-  if (desk_->is_active()) {
+  // Note that the desk may have already been destroyed.
+  if (desk_ && desk_->is_active()) {
     node_data->AddStringAttribute(
         ax::mojom::StringAttribute::kValue,
         l10n_util::GetStringUTF8(