aura: explicitly delete WindowPort in destructor

This way the WindowPort can reach back into the Window during destruction.

BUG=847992
TEST=covered by tests

Change-Id: I70df1c9512086f9e834ab76fa430c94f5723f34d
Reviewed-on: https://chromium-review.googlesource.com/1175133Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583163}

ui/aura/window.cc

@@ -93,9 +93,6 @@ Window::Window(WindowDelegate* delegate,
 Window::~Window() {
   WindowOcclusionTracker::ScopedPauseOcclusionTracking pause_occlusion_tracking;
 
-  // See comment in header as to why this is done.
-  std::unique_ptr<WindowPort> port = std::move(port_owner_);
-
   if (layer()->owner() == this)
     layer()->CompleteAllAnimations();
   layer()->SuppressPaint();
@@ -154,6 +151,11 @@ Window::~Window() {
   // acquired it.
   layer()->set_delegate(NULL);
   DestroyLayer();
+
+  // Delete the WindowPort now, in case it needs to reach back into the Window
+  // during destruction.
+  port_owner_.reset();
+  port_ = nullptr;
 }
 
 void Window::Init(ui::LayerType layer_type) {