Add null check to AXNodeObject::ComputeAriaOwnsChildren after AOM change

In change r484950 (https://codereview.chromium.org/2967193003),
AXNodeObject::ComputeAriaOwnsChildren was modified to handle both
ARIA attributes and AOM properties.

The change resulted in calling AxObjectCache().UpdateAriaOwns() with an
empty vector in a case where it wasn't called before, and when calling it
on an element with no Node (like on an anonymous block, for example),
that led to a crash. Fix that by only calling it when GetNode() is valid.

Bug: 740375
Change-Id: I058c9760e600e3e582778788a12950cbbeeb77a9
Reviewed-on: https://chromium-review.googlesource.com/574765Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487978}

third_party/WebKit/Source/modules/accessibility/AXNodeObject.cpp

@@ -2580,7 +2580,8 @@ void AXNodeObject::ComputeAriaOwnsChildren(
   Vector<String> id_vector;
   if (!CanHaveChildren() || IsNativeTextControl() ||
       HasContentEditableAttributeSet()) {
-    AxObjectCache().UpdateAriaOwns(this, id_vector, owned_children);
+    if (GetNode())
+      AxObjectCache().UpdateAriaOwns(this, id_vector, owned_children);
     return;
   }