mash: Don't crash when dragging a window to the side.

WindowServer::GetTreeWithId can be null in the case of
|in_nonclient_area|. It looks like |tree| was used without check in
4d3588cb.

BUG=630402

Review-Url: https://codereview.chromium.org/2170053003
Cr-Commit-Position: refs/heads/master@{#407181}

services/ui/ws/window_manager_state.cc

@@ -496,6 +496,7 @@ ClientSpecificId WindowManagerState::GetEventTargetClientId(
       tree = window_server()->GetTreeWithId(window->id().client_id);
   }
 
+  if (tree) {
     const ServerWindow* embed_root =
         tree->HasRoot(window) ? window : GetEmbedRoot(window);
     while (tree && tree->embedder_intercepts_events()) {
@@ -503,6 +504,7 @@ ClientSpecificId WindowManagerState::GetEventTargetClientId(
       tree = window_server()->GetTreeWithId(embed_root->id().client_id);
       embed_root = GetEmbedRoot(embed_root);
     }
+  }
 
   if (!tree) {
     DCHECK(in_nonclient_area);