Add fuzzer for AXTree and fix a couple of bugs it found.

ui/accessibility/BUILD.gn

Index: ui/accessibility/BUILD.gn
diff --git a/ui/accessibility/BUILD.gn b/ui/accessibility/BUILD.gn
index 4c03ce91d34e55a5f9e7465509fafdb339a30adc..f1c43b744758e9a45e9be729276f8a0a0962daae 100644
--- a/ui/accessibility/BUILD.gn
+++ b/ui/accessibility/BUILD.gn
@@ -6,6 +6,7 @@ import("//build/config/linux/pkg_config.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//build/json_schema_api.gni")
+import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 if (is_android) {
   import("//build/config/android/rules.gni")
@@ -153,3 +154,13 @@ json_schema_api("ax_gen") {
   root_namespace = "ui"
   schemas = true
 }
+
+fuzzer_test("ax_tree_fuzzer") {
+  sources = [
+    "ax_tree_fuzzer.cc",
+  ]
+
+  deps = [
+    ":accessibility",
+  ]
+}