Use event_data.source_id instead of event.id

Fixes msan issue warning of use of uninitialized variable.

==1066560==WARNING: MemorySanitizer: use-of-uninitialized-value

Bug: None
Test: ran unit tests with msan
Change-Id: Ia8e57144e051f639d234d9117e05c6f08236cf23
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468437
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817010}

chromecast/browser/accessibility/flutter/ax_tree_source_flutter.cc

@@ -266,7 +266,8 @@ void AXTreeSourceFlutter::NotifyAccessibilityEvent(
 
   if (event_data->event_type() ==
       gallium::castos::OnAccessibilityEventRequest_EventType_CONTENT_CHANGED) {
-    current_tree_serializer_->InvalidateSubtree(GetFromId(event.id));
+    current_tree_serializer_->InvalidateSubtree(
+        GetFromId(event_data->source_id()));
   }
 
   if (event_data->event_type() !=
@@ -294,8 +295,8 @@ void AXTreeSourceFlutter::NotifyAccessibilityEvent(
     HandleRoutes(&event_bundle.events);
 
     event_bundle.updates.emplace_back();
-    current_tree_serializer_->SerializeChanges(GetFromId(event.id),
-                                               &event_bundle.updates.back());
+    current_tree_serializer_->SerializeChanges(
+        GetFromId(event_data->source_id()), &event_bundle.updates.back());
 
     HandleLiveRegions(&event_bundle.events);