Wire up an api to darken screen for accessibility

chrome/browser/resources/chromeos/chromevox/chromevox/background/keymaps/next_keymap.json

 {
   "bindings": [
     {
       "command": "previousObject",
       "sequence": {
         "cvoxModifier": true,
         "keys": {
           "keyCode": [37]
         }
       }
@@ skipping 877 matching lines @@
     },
     {
       "command": "nextGroup",
       "sequence": {
         "cvoxModifier": true,
         "keys": {
           "keyCode": [40],
           "ctrlKey": [true]
         }
       }
+    },
+    {
+      "command": "darkenScreen",
+      "sequence": {
+        "cvoxModifier": true,
+        "keys": {
+          "keyCode": [118],
+          "shiftKey": [true]
+        }
+      }
+    },
+        {
+      "command": "undarkenScreen",
+      "sequence": {
+        "cvoxModifier": true,
+        "keys": {
+          "keyCode": [118]
+        }
+      }
     }
   ]
 }