AX: Add a null check for GetNode() in ComputeAccessibilityIsIgnored.

Since we pass this node as a reference, we need to ensure that it isn't
null.

This is a speculative fix for the referenced bug.

R=aleventhal@chromium.org
Bug: 1156599

Change-Id: I117910cd1304c61f7accea1b70fca92c75ece647
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2579943Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834853}

third_party/blink/renderer/modules/accessibility/ax_node_object.cc

@@ -471,6 +471,11 @@ bool AXNodeObject::ComputeAccessibilityIsIgnored(
   DCHECK(initialized_);
 #endif
 
+  // If we don't have a node, then ignore the node object.
+  // TODO(vmpstr/aleventhal): Investigate how this can happen.
+  if (!GetNode())
+    return true;
+
   // All nodes must have an unignored parent within their tree under
   // kRootWebArea, so force kRootWebArea to always be unignored.
   if (role_ == ax::mojom::blink::Role::kRootWebArea)