Add WorkspaceObserver to observe changes in workspace state

ui/aura_shell/workspace/workspace.cc

Index: ui/aura_shell/workspace/workspace.cc
diff --git a/ui/aura_shell/workspace/workspace.cc b/ui/aura_shell/workspace/workspace.cc
index cf762f34edd74622d053ffc2089d9c695836a600..7c9245b34cc2087d8252e91c861ad874c59d853c 100644
--- a/ui/aura_shell/workspace/workspace.cc
+++ b/ui/aura_shell/workspace/workspace.cc
@@ -180,6 +180,7 @@ void Workspace::Activate() {
 }
 
 void Workspace::Layout(aura::Window* ignore, aura::Window* no_animation) {
+  workspace_manager_->set_layout_in_progress(true);
   gfx::Rect work_area = workspace_manager_->GetWorkAreaBounds(bounds_);
   int total_width = GetTotalWindowsWidth();
   if (total_width < work_area.width()) {
@@ -210,6 +211,7 @@ void Workspace::Layout(aura::Window* ignore, aura::Window* no_animation) {
                    no_animation != windows_[1]);
     }
   }
+  workspace_manager_->set_layout_in_progress(false);
 }
 
 bool Workspace::ContainsFullscreenWindow() const {