Fixes crash in view_manager_lib

~ViewManagerSynchronizer destroys the view_manager_. Destroying the
view_manager_ may add transactions to pending_transactions_. So, we
have to make sure view_manager_ is destroyed first.

BUG=365012
TEST=none
R=ben@chromium.org
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/320713003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275730 0039d316-1c4b-4281-b951-d872f2087c98

mojo/services/public/cpp/view_manager/lib/view_manager_synchronizer.cc

@@ -447,6 +447,9 @@ ViewManagerSynchronizer::ViewManagerSynchronizer(ViewManagerDelegate* delegate)
 }
 
 ViewManagerSynchronizer::~ViewManagerSynchronizer() {
+  // Destroying the |view_manager_| may attempt to add transactions to
+  // |pending_transactions_|. So we need to destroy |view_manager_| first.
+  view_manager_.reset();
 }
 
 TransportNodeId ViewManagerSynchronizer::CreateViewTreeNode() {