Accessibility manager shouldn't try to access ash Shelf from login screen.

BUG=597000

Review URL: https://codereview.chromium.org/1837053003

Cr-Commit-Position: refs/heads/master@{#383847}

chrome/browser/chromeos/accessibility/accessibility_manager.cc

@@ -1194,6 +1194,11 @@ void AccessibilityManager::SetProfile(Profile* profile) {
   UpdateFocusHighlightFromPref();
   UpdateSelectToSpeakFromPref();
   UpdateSwitchAccessFromPref();
+
+  // Update the panel height in the shelf layout manager when the profile
+  // changes, since the shelf layout manager doesn't exist in the login profile.
+  if (chromevox_panel_)
+    chromevox_panel_->UpdatePanelHeight();
 }
 
 void AccessibilityManager::ActiveUserChanged(const AccountId& account_id) {
@@ -1426,9 +1431,14 @@ void AccessibilityManager::OnChromeVoxPanelClosing() {
   aura::Window* root_window = chromevox_panel_->GetRootWindow();
   chromevox_panel_widget_observer_.reset(nullptr);
   chromevox_panel_ = nullptr;
-  ash::Shelf::ForWindow(root_window)
-      ->shelf_layout_manager()
-      ->SetChromeVoxPanelHeight(0);
+
+  ash::Shelf* shelf = ash::Shelf::ForWindow(root_window);
+  if (!shelf)
+    return;
+
+  ash::ShelfLayoutManager* shelf_layout_manager = shelf->shelf_layout_manager();
+  if (shelf_layout_manager)
+    shelf_layout_manager->SetChromeVoxPanelHeight(0);
 }
 
 void AccessibilityManager::OnChromeVoxPanelDestroying() {

chrome/browser/chromeos/accessibility/chromevox_panel.cc

@@ -109,9 +109,17 @@ void ChromeVoxPanel::Close() {
 
 void ChromeVoxPanel::DidFirstVisuallyNonEmptyPaint() {
   widget_->Show();
-  ash::Shelf::ForWindow(GetRootWindow())
-      ->shelf_layout_manager()
-      ->SetChromeVoxPanelHeight(kPanelHeight);
+  UpdatePanelHeight();
+}
+
+void ChromeVoxPanel::UpdatePanelHeight() {
+  ash::Shelf* shelf = ash::Shelf::ForWindow(GetRootWindow());
+  if (!shelf)
+    return;
+
+  ash::ShelfLayoutManager* shelf_layout_manager = shelf->shelf_layout_manager();
+  if (shelf_layout_manager)
+    shelf_layout_manager->SetChromeVoxPanelHeight(kPanelHeight);
 }
 
 void ChromeVoxPanel::EnterFullscreen() {

chrome/browser/chromeos/accessibility/chromevox_panel.h

@@ -31,6 +31,7 @@ class ChromeVoxPanel : public views::WidgetDelegate,
 
   void Close();
   void DidFirstVisuallyNonEmptyPaint();
+  void UpdatePanelHeight();
   void EnterFullscreen();
   void ExitFullscreen();
   void DisableSpokenFeedback();