Split the private webstore install API into two parts....

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 4221 matching lines @@
           },
           {
             "name": "callback",
             "type": "function",
             "optional": "true",
             "parameters": []
           }
         ]
       },
       {
+        "name": "beginInstall",
+        "description": "Initiates the install process for the given extension id",
+        "parameters": [
+          {
+            "name": "expected_id",
+            "type": "string",
+            "description": "The id of the extension to be installled. Must be called during a user gesture."
+          },
+          {
+            "name": "callback",
+            "type": "function",
+            "optional": "true",
+            "parameters": []
+          }
+        ]
+      },
+      {
+        "name": "completeInstall",
+        "description": "",
+        "parameters": [
+          {
+            "name": "expected_id",
+            "type": "string",
+            "description": "The id of the extension to be installled. This should match a previous call to beginInstall."
+          },
+          {
+            "name": "callback",
+            "type": "function",
+            "optional": "true",
+            "parameters": []
+          }
+        ]
+      },
+      {
         "name": "getBrowserLogin",
         "description": "Returns the logged-in sync user login if there is one, or the empty string otherwise.",
         "parameters": [
           {
             "name": "callback",
             "type": "function",
             "optional": "false",
             "parameters": [
               {
                 "name": "info",
@@ skipping 51 matching lines @@
                   "token": { "type": "string", "optional": true }
                 }
               }
             ]
           }
         ]
       }
     ]
   }
 ]