Plumbs checkedStateDescription to AutomationNode

Checked state description is not read out because I missed adding
checked state description to automation_node.js in CL:2444875 .
Add it to plumb checked state description to AutomationNode.

Bug: b:154433831
Test: manual (run locally modified arc.AccessibilitySpeech.vm)
Change-Id: Id56e0b9229efcfff9dbda13aee542567e86a703c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2528515Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Saho Kobayashi <sahok@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827148}

extensions/renderer/resources/automation/automation_node.js

@@ -1301,6 +1301,7 @@ var stringAttributes = [
     'accessKey',
     'ariaInvalidValue',
     'autoComplete',
+    'checkedStateDescription',
     'className',
     'containerLiveRelevant',
     'containerLiveStatus',