Allocate AXTreeDelegate before AXTree in ax_tree_fuzzer.

This was causing a stack-use-after-scope when fuzzing. Manually confirmed
that allocating the delegate first fixes the problem on the input attached
to the bug.

BUG=688104

Review-Url: https://codereview.chromium.org/2715663006
Cr-Commit-Position: refs/heads/master@{#452760}

ui/accessibility/ax_tree_fuzzer.cc

@@ -43,8 +43,8 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data, size_t size) {
   // Run with --v=1 to aid in debugging a specific crash.
   VLOG(1) << "Input accessibility tree:\n" << initial_state.ToString();
 
-  ui::AXTree tree;
   EmptyAXTreeDelegate delegate;
+  ui::AXTree tree;
   tree.SetDelegate(&delegate);
   tree.Unserialize(initial_state);