Add private system extension API to get system update status

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
           "tab": {"$ref": "Tab", "optional": true, "description":"This property will <b>only</b> be present when the connection was opened from a tab or content script."},
@@ skipping 507 matching lines @@
       },
       {
         "name": "onWokeUp",
         "type": "function",
         "description": "Fired when the device wakes up from sleep.",
         "parameters": []
       }
     ]
   },
   {
+    "namespace": "accessibilityPrivate",
+    "nodoc": true,
+    "platforms": ["chromeos"],
+    "types": [
+      {
+        "id": "UpdateStatus",
+        "type": "object",
+        "description": "Information about the system update.",
+        "properties": {
+          "state": {
+            "type": "string",
+            "enum": ["NotAvailable", "Updating", "NeedReboot"],
+            "description": "State of system update."

[dmazzoni, 2011/11/30 15:46:04]

Document briefly what these mean. (For example, make it clear that NeedReboot
means that the update has been downloaded and installed and will take effect as
soon as the user reboots.)

What if there's an error retrieving the update status? What happens?

[hashimoto, 2011/12/01 02:28:00]

Done.
Currently 'NotAvailable' is used for error state because the purpose of this API
is to notify users about available update, not about the detail of the update
mechanism.

+          },
+          "download_progress": {
+            "type": "number",
+            "description": "Pregress of download. [0.0 - 1.0]"

[dmazzoni, 2011/11/30 15:46:04]

Document that this will be 0 if the update is NotAvailable and 1 if the update
is NeedReboot.

[hashimoto, 2011/12/01 02:28:00]

Done.

+          }
+        }
+      }
+    ],
+    "functions": [
+      {
+        "name": "getUpdateStatus",
+        "type": "function",
+        "description": "Gets information about the system update.",
+        "parameters": [
+          {
+            "type": "function",
+            "name": "callback",
+            "parameters": [
+              {
+                "$ref": "UpdateStatus",
+                "name": "status",
+                "description": "Details of the system update"
+              }
+            ]
+          }
+        ]
+      }
+    ],
+    "events": []
+  },
+  {
     "namespace": "experimental.speechInput",
     "types": [
       {
         "id": "SpeechInputStartOptions",
         "type": "object",
         "description": "Object describing the options used for speech recognition.",
         "properties": {
           "language": {
             "type": "string",
             "optional": true,
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                   }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]