Tolerate GetRoot() returning null.

content/browser/accessibility/browser_accessibility_manager_win.cc

Index: content/browser/accessibility/browser_accessibility_manager_win.cc
diff --git a/content/browser/accessibility/browser_accessibility_manager_win.cc b/content/browser/accessibility/browser_accessibility_manager_win.cc
index bf70c576ad676d7f3e83c67ee2bb6466a6a08740..5ee08f0fae5d589717997fbdf4d784b000b52be5 100644
--- a/content/browser/accessibility/browser_accessibility_manager_win.cc
+++ b/content/browser/accessibility/browser_accessibility_manager_win.cc
@@ -150,13 +150,14 @@ void BrowserAccessibilityManagerWin::OnWindowFocused() {
 
   // Try to fire a focus event on the root first and then the focused node.
   // This will clear focus_event_on_root_needed_ if successful.
-  if (focus_ != tree_->root())
+  if (focus_ != tree_->root() && GetRoot())
     NotifyAccessibilityEvent(ui::AX_EVENT_FOCUS, GetRoot());
   BrowserAccessibilityManager::OnWindowFocused();
 }
 
 void BrowserAccessibilityManagerWin::UserIsReloading() {
-  MaybeCallNotifyWinEvent(IA2_EVENT_DOCUMENT_RELOAD, GetRoot());
+  if (GetRoot())
+    MaybeCallNotifyWinEvent(IA2_EVENT_DOCUMENT_RELOAD, GetRoot());
 }
 
 void BrowserAccessibilityManagerWin::NotifyAccessibilityEvent(
@@ -253,6 +254,9 @@ void BrowserAccessibilityManagerWin::NotifyAccessibilityEvent(
       break;
   }
 
+  if (!node)
+    return;
+
   if (event_id != EVENT_MIN) {
     // Pass the node's unique id in the |child_id| argument to NotifyWinEvent;
     // the AT client will then call get_accChild on the HWND's accessibility