athena: Fix a use-after-free case.

|split_view_controller_| depends on |mru_window_tracker_|. So destroy them
in the correct order to avoid the use-after-free when destroying the window
manager.

BUG=none
TBR=mfomitchev@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/471803002

Cr-Commit-Position: refs/heads/master@{#289464}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289464 0039d316-1c4b-4281-b951-d872f2087c98

athena/wm/window_manager_impl.cc

@@ -135,6 +135,7 @@ WindowManagerImpl::WindowManagerImpl() {
 
 WindowManagerImpl::~WindowManagerImpl() {
   overview_.reset();
+  split_view_controller_.reset();
   mru_window_tracker_.reset();
   if (container_) {
     container_->RemoveObserver(this);