Prune child trees of invisible aura windows

R=anastasi@google.com, dmazzoni@chromium.org

Test: manually verify the virtual keyboard is pruned from the accessibility tree when hidden and attached when shown
Change-Id: Idb5a28e7458a18b8284d455976be48bf90ec53da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375941Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801576}

ui/views/accessibility/ax_window_obj_wrapper.cc

@@ -144,8 +144,12 @@ void AXWindowObjWrapper::Serialize(ui::AXNodeData* out_node_data) {
     //
     // To avoid this double-parenting, only add the child tree ID of this
     // window if the top-level window doesn't have an associated Widget.
+    //
+    // Also, if this window is not visible, its child tree should also be
+    // non-visible so prune it.
     if (!window_->GetToplevelWindow() ||
-        GetWidgetForWindow(window_->GetToplevelWindow())) {
+        GetWidgetForWindow(window_->GetToplevelWindow()) ||
+        !window_->IsVisible()) {
       return;
     }