[fuchsia] Handle receiving multiple accessibility trees

A WebContentsObserver may receive updates from multiple AXTrees.
Currently, that isn't supported, so add a fix that ignores updates that
aren't from the first tree that the WebContentsObserver receives.

Test: AccessibilityBridgeTest.MultipleAXTrees
Bug: 1136541
Change-Id: If1fb3de647c150c58bc07b55477452f91abfcef7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2469470
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820488}

fuchsia/engine/browser/accessibility_bridge.cc

@@ -87,13 +87,19 @@ void AccessibilityBridge::OnCommitComplete() {
 
 void AccessibilityBridge::AccessibilityEventReceived(
     const content::AXEventNotificationDetails& details) {
-  if (!enable_semantic_updates_) {
-    // No need to process events if Fuchsia is not receiving them.
+  // No need to process events if Fuchsia is not receiving them.
+  if (!enable_semantic_updates_)
     return;
-  }
 
   // Updates to AXTree must be applied first.
   for (const ui::AXTreeUpdate& update : details.updates) {
+    if (!update.has_tree_data &&
+        ax_tree_.GetAXTreeID() != ui::AXTreeIDUnknown() &&
+        ax_tree_.GetAXTreeID() != details.ax_tree_id) {
+      // TODO(https://crbug.com/1128954): Add support for combining AXTrees.
+      continue;
+    }
+
     if (!ax_tree_.Unserialize(update)) {
       // If this fails, it is a fatal error that will cause an early exit.
       std::move(on_error_callback_).Run(ZX_ERR_INTERNAL);