A11y: fix reference counting leak on AtkComponent ref_at_point.

The call to ref_at_point was allocating the extra reference
for transferring ref ownership as requested by the method.

Change-Id: I76ed65d75cc136fff7dbaf9c61fad3e3a59f2c47
Reviewed-on: https://chromium-review.googlesource.com/794138Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: José Dapena Paz <jose.dapena@lge.com>
Cr-Commit-Position: refs/heads/master@{#520228}

ui/accessibility/platform/ax_platform_node_auralinux.cc

@@ -288,7 +288,10 @@ static AtkObject* ax_platform_node_auralinux_ref_accessible_at_point(
   if (!obj)
     return nullptr;
 
-  return obj->HitTestSync(x, y, coord_type);
+  AtkObject* result = obj->HitTestSync(x, y, coord_type);
+  if (result)
+    g_object_ref(result);
+  return result;
 }
 
 static gboolean ax_platform_node_auralinux_grab_focus(