Pause recalculation of aura window occlusion during reordering.

BUG=1111587

Change-Id: I34db79f24ecfb314687edd24d13c69434a0542ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333878Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Wei Li <weili@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794303}

ui/views/widget/window_reorderer.cc

@@ -14,6 +14,7 @@
 #include "base/containers/adapters.h"
 #include "base/macros.h"
 #include "ui/aura/window.h"
+#include "ui/aura/window_occlusion_tracker.h"
 #include "ui/views/view.h"
 #include "ui/views/view_constants_aura.h"
 
@@ -160,6 +161,8 @@ void WindowReorderer::ReorderChildWindows() {
 
   std::vector<ui::Layer*> children_layer_order;
 
+  aura::WindowOcclusionTracker::ScopedPause pause_occlusion_tracking;
+
   // For the sake of simplicity, reorder both the layers owned by views and the
   // layers of windows associated with a view. Iterate through
   // |view_with_layer_order| backwards and stack windows at the bottom so that