Made AXNode::IsDescendantOf work the same across BrowserAccessibility,...

Made AXNode::IsDescendantOf work the same across BrowserAccessibility, AXPlatformNodeBase and AXNode

An inconsistency in the behavior of this method that
I found while merging BrowserAccessibilityPosition and AXNodePosition.

AX-Relnotes: n/a.

R=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I34ce5d10bc67a7b78438e94a4a9c4b6c21fd5bc9
Bug: 1049261
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644620Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846221}

ui/accessibility/ax_node.cc

@@ -4,10 +4,11 @@
 
 #include "ui/accessibility/ax_node.h"
 
+#include <string.h>
+
 #include <algorithm>
 #include <utility>
 
-#include "base/strings/string16.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
@@ -417,6 +418,9 @@ void AXNode::Destroy() {
 }
 
 bool AXNode::IsDescendantOf(const AXNode* ancestor) const {
+  if (!ancestor)
+    return false;
+
   if (this == ancestor)
     return true;
   if (parent())
@@ -1346,13 +1350,12 @@ bool AXNode::IsInListMarker() const {
   if (data().role == ax::mojom::Role::kListMarker)
     return true;
 
-  // List marker node's children can only be text elements.
+  // The children of a list marker node can only be text nodes.
   if (!IsText())
     return false;
 
-  // There is no need to iterate over all the ancestors of the current anchor
-  // since a list marker node only has children on 2 levels.
-  // i.e.:
+  // There is no need to iterate over all the ancestors of the current node
+  // since a list marker has descendants that are only 2 levels deep, i.e.:
   // AXLayoutObject role=kListMarker
   // ++StaticText
   // ++++InlineTextBox

ui/accessibility/ax_node.h

@@ -13,6 +13,7 @@
 #include <vector>
 
 #include "base/optional.h"
+#include "base/strings/string16.h"
 #include "build/build_config.h"
 #include "ui/accessibility/ax_export.h"
 #include "ui/accessibility/ax_node_data.h"