Fix crash when clicking on slide-out settings button of ARC notification.

Opening the inline settings now triggers another surface created event, which
is not currently expected by the ArcAccessibilityHelperBridge. If the
corresponding tree already exists, then we should just return early.

TEST=manually verified
BUG=1042554

Change-Id: Ic689184f54c4a737ddd35d7ed05e42b09d3c21bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003197Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734702}

chrome/browser/chromeos/arc/accessibility/arc_accessibility_helper_bridge.cc

@@ -360,6 +360,9 @@ void ArcAccessibilityHelperBridge::OnNotificationStateChanged(
   auto key = KeyForNotification(notification_key);
   switch (state) {
     case arc::mojom::AccessibilityNotificationStateType::SURFACE_CREATED: {
+      if (GetFromKey(key))
+        return;
+
       AXTreeSourceArc* tree_source = CreateFromKey(std::move(key));
       ui::AXTreeData tree_data;
       if (!tree_source->GetTreeData(&tree_data)) {