Wire up an api to darken screen for accessibility

chrome/browser/resources/chromeos/chromevox/strings/chromevox_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <grit base_dir="." current_release="1" latest_public_release="0"
       output_all_resource_defines="false" enc_check="möl" source_lang_id="en">
   <outputs>
     <output filename="_locales/am/messages.json" type="chrome_messages_json" lang="am"/>
     <output filename="_locales/ar/messages.json" type="chrome_messages_json" lang="ar"/>
     <output filename="_locales/bg/messages.json" type="chrome_messages_json" lang="bg"/>
     <output filename="_locales/bn/messages.json" type="chrome_messages_json" lang="bn"/>
     <output filename="_locales/ca/messages.json" type="chrome_messages_json" lang="ca"/>
@@ skipping 2690 matching lines @@
       </message>
       <message desc="Describes an audio clip that gets played for a specific event or control type. Use the default string as a guide to what the audio clip represents or conveys." name="IDS_CHROMEVOX_POP_UP_BUTTON_EARCON_DESCRIPTION">
         A pop up button
       </message>
       <message desc="Describes an audio clip that gets played for a specific event or control type. Use the default string as a guide to what the audio clip represents or conveys." name="IDS_CHROMEVOX_SLIDER_EARCON_DESCRIPTION">
         A slider
       </message>
       <message desc="Describes an audio clip that gets played for a specific event or control type. Use the default string as a guide to what the audio clip represents or conveys." name="IDS_CHROMEVOX_WRAP_EARCON_DESCRIPTION">
         Wrap from beginning to end or end to beginning inside of a page, dialog, or other container
       </message>
+      <message desc="Describes a command to darken the screen." name="IDS_CHROMEVOX_DARKEN_SCREEN">

[dmazzoni, 2017/03/24 16:02:40]

Maybe explain it in case "darken" isn't clear. How about: Describes a command to
make the screen black.

[David Tseng, 2017/03/24 18:07:43]

How about turns the visual display on/off?

[michaelpg, 2017/03/24 21:13:41]

The string itself seems ambiguous too; I would expect "darken screen" to mean
"incrementally dim the screen". Are these set in stone or would "Turn off
screen/Turn on screen" work? (As a side note, "turn on" and "turn off" are hard
to translate, but the description makes it clear.)

[dmazzoni, 2017/03/24 21:24:37]

Sounds good

+        Darken screen
+      </message>
+      <message desc="Describes a command to undarken the screen." name="IDS_CHROMEVOX_UNDARKEN_SCREEN">
+        Undarken screen
+      </message>
     </messages>
   </release>
 </grit>