Minor refactor of ShelfWindowWatcher

Get rid of an unused variable, and properly use OnWindowAdded()
instead of OnWindowHierarchyChanged() which can be triggered for
many events we're not interested in.

BUG=997102

Change-Id: If9f7677bdb6c7f73ddcfa455139149e4d7922ce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2223367Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773739}

ash/shelf/shelf_window_watcher.cc

@@ -17,6 +17,7 @@
 #include "base/strings/string_util.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/base/resource/resource_bundle.h"
+#include "ui/display/types/display_constants.h"
 #include "ui/resources/grit/ui_resources.h"
 #include "ui/wm/public/activation_client.h"
 
@@ -69,17 +70,12 @@ ShelfWindowWatcher::ContainerWindowObserver::ContainerWindowObserver(
 ShelfWindowWatcher::ContainerWindowObserver::~ContainerWindowObserver() =
     default;
 
-// static
-const char ShelfWindowWatcher::kDefaultShelfIdPrefix[] = "ShelfWindowWatcher";
-
-void ShelfWindowWatcher::ContainerWindowObserver::OnWindowHierarchyChanged(
-    const HierarchyChangeParams& params) {
-  if (!params.old_parent && params.new_parent &&
-      desks_util::IsDeskContainer(params.new_parent)) {
-    // A new window was created in one of the desks' containers. Note that the
-    // shelf is globally showing all apps from all active and inactive desks.
-    window_watcher_->OnUserWindowAdded(params.target);
-  }
+void ShelfWindowWatcher::ContainerWindowObserver::OnWindowAdded(
+    aura::Window* new_window) {
+  DCHECK(new_window);
+  DCHECK(new_window->parent());
+  DCHECK(desks_util::IsDeskContainer(new_window->parent()));
+  window_watcher_->OnUserWindowAdded(new_window);
 }
 
 void ShelfWindowWatcher::ContainerWindowObserver::OnWindowDestroying(

ash/shelf/shelf_window_watcher.h

@@ -27,8 +27,6 @@ class ShelfWindowWatcher : public ::wm::ActivationChangeObserver,
   explicit ShelfWindowWatcher(ShelfModel* model);
   ~ShelfWindowWatcher() override;
 
-  static const char kDefaultShelfIdPrefix[];
-
  private:
   // Observes for windows being added to a root window's default container.
   class ContainerWindowObserver : public aura::WindowObserver {
@@ -38,7 +36,7 @@ class ShelfWindowWatcher : public ::wm::ActivationChangeObserver,
 
    private:
     // aura::WindowObserver:
-    void OnWindowHierarchyChanged(const HierarchyChangeParams& params) override;
+    void OnWindowAdded(aura::Window* new_window) override;
     void OnWindowDestroying(aura::Window* window) override;
 
     ShelfWindowWatcher* window_watcher_;