AXSubrole and AXRoleDescription is not exposed for HTML form element.

content/app/strings/content_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- This file contains definitions of resources that will be translated for
 each locale. Specifically, these are UI strings that are used by content that
 need to be translated for each locale.-->
 
 <!-- Some of these strings and string descriptions were taken from
 WebKit/win/WebCoreLocalizedStrings.cpp so we include the original license
 below:
 
@@ skipping 364 matching lines @@
       </if>
       <message name="IDS_AX_ROLE_ADDRESS" desc="accessibility role description for an address (as in a person's contact information)">
         address
       </message>
       <message name="IDS_AX_ROLE_MAIN_CONTENT" desc="accessibility role description for main content of the document.">
         main
       </message>
       <message name="IDS_AX_ROLE_NAVIGATIONAL_LINK" desc="accessibility role description for group of navigational links.">
         navigation
       </message>
+      <message name="IDS_AX_ROLE_FORM" desc="accessibility role description for form">
+        form
+      </message>
 
       <message name="IDS_AX_BUTTON_ACTION_VERB" desc="Verb stating the action that will occur when a button is pressed, as used by accessibility.">
         press
       </message>
       <message name="IDS_AX_RADIO_BUTTON_ACTION_VERB" desc="Verb stating the action that will occur when a radio button is clicked, as used by accessibility.">
         select
       </message>
       <message name="IDS_AX_TEXT_FIELD_ACTION_VERB" desc="Verb stating the action that will occur when a text field is selected, as used by accessibility.">
         activate
       </message>
@@ skipping 323 matching lines @@
         Please lengthen this text to <ph name="MIN_CHARACTERS">$2<ex>101</ex></ph> characters or more (you are currently using <ph name="CURRENT_LENGTH">$1<ex>100</ex></ph> characters).
       </message>
 
       <message name="IDS_PLUGIN_INITIALIZATION_ERROR" desc="A message displayed when a plug-in failed to load">
         Couldn't load plug-in.
       </message>
 
     </messages>
   </release>
 </grit>