Fix AXRemoteHost::SendEvent DCHECK when toggling ChromeVox with KSV app

Bug: 910224
Change-Id: Ie1d13e6d2c41b4096ce7912ba494db2fe468c678
Reviewed-on: https://chromium-review.googlesource.com/c/1355196Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612350}

ui/views/mus/ax_remote_host.cc

@@ -226,12 +226,12 @@ void AXRemoteHost::Disable() {
 void AXRemoteHost::SendEvent(AXAuraObjWrapper* aura_obj,
                              ax::mojom::Event event_type) {
   DCHECK(aura_obj);
-  if (!enabled_ || !widget_)
+  // Early return when this host is disabled or only partially initialized.
+  // This roughly matches the behavior in AutomationManagerAura::SendEvent.
+  // Toggling ChromeVox off does not disable the host, etc: crbug.com/910224
+  if (!enabled_ || !widget_ || !tree_serializer_ || !tree_source_)
     return;
 
-  DCHECK(tree_source_);
-  DCHECK(tree_serializer_);
-
   ui::AXTreeUpdate update;
   if (!tree_serializer_->SerializeChanges(aura_obj, &update)) {
     LOG(ERROR) << "Unable to serialize accessibility tree.";