Add null check when reporting changed nodes to an AX tree.

ui/accessibility/ax_tree.cc

Index: ui/accessibility/ax_tree.cc
diff --git a/ui/accessibility/ax_tree.cc b/ui/accessibility/ax_tree.cc
index b2c0833204d2c7a415cda0442a467b8406b3a531..807ca8f826c332db13d716720e06e651150538d1 100644
--- a/ui/accessibility/ax_tree.cc
+++ b/ui/accessibility/ax_tree.cc
@@ -220,6 +220,9 @@ bool AXTree::Unserialize(const AXTreeUpdate& update) {
     changes.reserve(update.nodes.size());
     for (size_t i = 0; i < update.nodes.size(); ++i) {
       AXNode* node = GetFromId(update.nodes[i].id);
+      if (!node)

[David Tseng, 2017/06/07 23:24:24]

At this point, I would have expected there to be pending nodes above? Why wasn't
this caught by the deserialization below?

[dmazzoni, 2017/06/19 17:12:24]

Imagine that B's parent is A, but B is invisible so
A does not list B as a child. We were unserializing
B and then A, in that order. B unserializes fine. Then
we get to A, and A does not list B as a child anymore,
so B gets deleted. Then later during postprocessing we
search for B but it's no longer in the tree.

It's a weird corner case but I don't think the
unserialization logic was wrong other than this
missing null check.

+        continue;
+
       bool is_new_node = new_nodes.find(node) != new_nodes.end();
       bool is_reparented_node =
           is_new_node && update_state.HasRemovedNode(node);