aura: makes ~WindowPortLocal get Env from window

Rather than Env::GetInstance(). This ensures the right instance is used.

BUG=847992
TEST=covered by tests

Change-Id: I9b75458c1e4d21402ef2e58833c57ff9a289b7a0
Reviewed-on: https://chromium-review.googlesource.com/1176062Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583296}

ui/aura/local/window_port_local.cc

@@ -67,8 +67,7 @@ WindowPortLocal::WindowPortLocal(Window* window)
 
 WindowPortLocal::~WindowPortLocal() {
   if (frame_sink_id_.is_valid()) {
-    auto* context_factory_private =
-        aura::Env::GetInstance()->context_factory_private();
+    auto* context_factory_private = window_->env()->context_factory_private();
     auto* host_frame_sink_manager =
         context_factory_private->GetHostFrameSinkManager();
     host_frame_sink_manager->InvalidateFrameSinkId(frame_sink_id_);