Set keys for SwitchAccessEventHandler to capture using accessibiltyPrivate API

chrome/common/extensions/api/accessibility_private.json

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "accessibilityPrivate",
     "compiler_options": {
       "implemented_in": "chrome/browser/accessibility/accessibility_extension_api.h"
     },
@@ skipping 82 matching lines @@
         "name": "darkenScreen",
         "type": "function",
         "description": "Darkens or undarkens the screen.",
         "parameters": [
           {
             "type": "boolean",
             "name": "enabled",
             "description": "True to darken screen; false to undarken screen."
           }
         ]
+      },
+      {
+        "name": "setSwitchAccessKeys",
+        "type": "function",
+        "description": "Change the keyboard keys captured by Switch Access.",
+        "parameters": [
+          {
+            "name": "key_codes",
+            "type": "array",
+            "items": { "type": "integer" },

[Devlin, 2017/05/30 19:22:03]

note: we support min and max values, so if we know these should always be
between 0 and 'z', we could hard-code those values here with a comment and it
would be checked js-side.

[elichtenberg, 2017/05/31 00:58:44]

Would it be better to just do this check in accessibility_extension_api.cc?
Because that way, we can get the key codes from headers, rather than hard coding
the key codes. Also, we might expand to other keys later, so just having a
minimum and maximum key code might not work later on.

[Devlin, 2017/05/31 01:33:32]

The advantage to having the minimum specified here is that it will be validated
synchronously in the renderer, so if someone tried to do
setSwitchAccessKeys([-1]), it would immediately throw an error.  This is often
nicer than throwing an error asynchronously in runtime.lastError, which is what
happens when we return the result from the browser process.  But, as you noted,
this means we can't import the exact values from the headers.  Given this is a
private API, I'm fine either way.

[elichtenberg, 2017/06/01 19:55:27]

Good points. I ended up setting min and max values.

+            "description": "The key codes for the keys that will be captured."
+          }
+        ]
       }
     ],
     "events": [
       {
         "name": "onIntroduceChromeVox",
         "type": "function",
         "description": "Fired whenever ChromeVox should output introduction."
       },
       {
         "name": "onAccessibilityGesture",
@@ skipping 14 matching lines @@
       },
       {
         "name": "onTwoFingerTouchStop",
         "type": "function",
         "description": "Fired when  the user is no longer holding down two fingers (including releasing one, holding down three, or moving them).",
         "parameters": []
       }
     ]
   }
 ]