Make blink::AXObject::AncestorsIterator STL compatible.

Commit 31e5188ffc9a04295997d22bfdb68fc367bef047, "Used some methods from
AXRoleProperties in AXObject" started using std::any_of with this custom
iterator type. Unfortunately this iterator does not provide traits
mandated by the standard. This works fine for libcxx, but fails when
compiling the code against libstdc++.

Bug: 819294
Change-Id: I78fe25475593d73ce255f1de955aa41e936dff86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207112
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Commit-Position: refs/heads/master@{#769713}

diff --git a/third_party/blink/renderer/modules/accessibility/ax_object.h b/third_party/blink/renderer/modules/accessibility/ax_object.h
index 841715b..fbb87bf 100644
--- a/third_party/blink/renderer/modules/accessibility/ax_object.h
+++ b/third_party/blink/renderer/modules/accessibility/ax_object.h
@@ -274,6 +274,12 @@
   class MODULES_EXPORT AncestorsIterator final
       : public GarbageCollected<AncestorsIterator> {
    public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = AXObject;
+    using difference_type = ptrdiff_t;
+    using pointer = value_type*;
+    using reference = value_type&;
+
     ~AncestorsIterator() = default;
 
     AncestorsIterator(const AncestorsIterator& other)