[a11y] Accessibility bridge rejects actions on invalidtrees.

This change rejects actions performed on invalid accessibility trees by
checking if the tree still exists.

Test: Does not crash during manual test of b/2736858
Bug: 1180440
Bug: b/2736858

(cherry picked from commit 1e82f57)

Change-Id: I12ca7b9ac53e0a026ce62b29b165f4cb169dd403
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2736858
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#859902}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2739912
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/branch-heads/4430@{#173}
Cr-Branched-From: e5ce7dc-refs/heads/master@{#857950}

fuchsia/engine/browser/accessibility_bridge.cc

@@ -176,9 +176,14 @@ void AccessibilityBridge::OnAccessibilityActionRequested(
     action_data.scroll_behavior = ax::mojom::ScrollBehavior::kScrollIfVisible;
   }
 
-  web_contents_->GetMainFrame()
-      ->FromAXTreeID(ax_id->first)
-      ->AccessibilityPerformAction(action_data);
+  auto* frame = web_contents_->GetMainFrame()->FromAXTreeID(ax_id->first);
+  if (!frame) {
+    // Fuchsia targeted a tree that does not exist.
+    callback(false);
+    return;
+  }
+
+  frame->AccessibilityPerformAction(action_data);
   callback(true);
 
   if (event_received_callback_for_test_) {