Set keys for SwitchAccessEventHandler to capture using accessibiltyPrivate API

chrome/common/extensions/api/accessibility_private.json

Index: chrome/common/extensions/api/accessibility_private.json
diff --git a/chrome/common/extensions/api/accessibility_private.json b/chrome/common/extensions/api/accessibility_private.json
index 0b9f629f1defebdc7057e17d586e91fc53bd6e91..9e4361c095d271bda5006302df334b7d9eccaaa6 100644
--- a/chrome/common/extensions/api/accessibility_private.json
+++ b/chrome/common/extensions/api/accessibility_private.json
@@ -100,6 +100,19 @@
             "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": [