Adding a gray semi transparent backdrop behind the topmost window within the default container

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index b80c60533086c7a5120c07cbee6be0c18d1e7a2a..ddae9d6270aa10f50b2a4d2cdfe023385385c831 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -382,6 +382,14 @@ void Shell::OnCastingSessionStartedOrStopped(bool started) {
 #endif
 }
 
+void Shell::OnOverviewModeStarted() {
+  FOR_EACH_OBSERVER(ShellObserver, observers_, OnOverviewModeStarted());
+}
+
+void Shell::OnOverviewModeEnded() {
+  FOR_EACH_OBSERVER(ShellObserver, observers_, OnOverviewModeEnded());
+}
+
 void Shell::CreateShelf() {
   RootWindowControllerList controllers = GetAllRootWindowControllers();
   for (RootWindowControllerList::iterator iter = controllers.begin();
@@ -437,6 +445,10 @@ void Shell::EnableMaximizeModeWindowManager(bool enable) {
   }
 }
 
+bool Shell::IsMaximizeModeWindowManagerEnabled() {
+  return NULL != maximize_mode_window_manager_.get();
+}
+
 void Shell::UpdateShelfVisibility() {
   RootWindowControllerList controllers = GetAllRootWindowControllers();
   for (RootWindowControllerList::iterator iter = controllers.begin();
@@ -651,6 +663,11 @@ Shell::~Shell() {
   // TooltipController is deleted with the Shell so removing its references.
   RemovePreTargetHandler(tooltip_controller_.get());
 
+  // Disable the maximize mode here since it can be destroyed without without
+  // side effects, but needs to be destroyed before the root windows and the
+  // WorkspaceLayoutManager get destroyed.
+  maximize_mode_window_manager_.reset();
+
   // AppList needs to be released before shelf layout manager, which is
   // destroyed with shelf container in the loop below. However, app list
   // container is now on top of shelf container and released after it.