Fix a reference to the accessibility object for auralinux

Bug: 959650
Change-Id: Iddaddd4de70a3fb80ca6b21a9d9e0a7238e06162
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715369Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Commit-Queue: Jessica Tallon <jtallon@igalia.com>
Cr-Commit-Position: refs/heads/master@{#680380}

ui/accessibility/platform/ax_platform_node_auralinux.cc

@@ -3856,7 +3856,7 @@ gchar* AXPlatformNodeAuraLinux::GetSelectionWithText(int* start_offset,
       selection_start == selection_end)
     return nullptr;
 
-  return atk_text::GetText(ATK_TEXT(atk_object_), selection_start,
+  return atk_text::GetText(ATK_TEXT(GetOrCreateAtkObject()), selection_start,
                            selection_end);
 }