exo: Fix tear down crash in Seat class.

Fix ExoParts destruction order to ensure that WMHelper instance
is valid when exo::Display is destroyed.

Bug: 788731
Test: Running Chrome with DCHECKs no longer fails at shutdown.
Change-Id: I9bddecd863d92b600378488e04612cff87cef1ae
Reviewed-on: https://chromium-review.googlesource.com/792214
Commit-Queue: David Reveman <reveman@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519523}

ash/wayland/wayland_server_controller.cc

@@ -57,6 +57,7 @@ WaylandServerController::CreateIfNecessary() {
 WaylandServerController::~WaylandServerController() {
   wayland_watcher_.reset();
   wayland_server_.reset();
+  display_.reset();
   exo::WMHelper::SetInstance(nullptr);
   wm_helper_.reset();
 }

chrome/browser/exo_parts.cc

@@ -150,6 +150,7 @@ std::unique_ptr<ExoParts> ExoParts::CreateIfNecessary() {
 ExoParts::~ExoParts() {
   wayland_watcher_.reset();
   wayland_server_.reset();
+  display_.reset();
   exo::WMHelper::SetInstance(nullptr);
   wm_helper_.reset();
 }