Disallowing copy & assign for ui::AXTree.

ui::AXTree contains owned pointers which it is responsible for freeing,
copying this is therefore non-trivial and we don't define our own copy
constructor.

R=aboxhall,dmazzoni

Change-Id: Iecbec9fc69c56dff96d3640f83abb6ef783ca1a5
Reviewed-on: https://chromium-review.googlesource.com/c/1358112
Commit-Queue: Chris Hall <chrishall@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613383}

ui/accessibility/ax_tree.h

@@ -374,6 +374,9 @@ class AX_EXPORT AXTree : public AXNode::OwnerTree {
   // is an element within an ordered set.
   // Invalidated every time the tree is updated.
   mutable std::unordered_map<int32_t, OrderedSetInfo> ordered_set_info_map_;
+
+  // AXTree owns pointers so copying is non-trivial.
+  DISALLOW_COPY_AND_ASSIGN(AXTree);
 };
 
 }  // namespace ui