Add a flag that lets the webstore show a different UI on app install.

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 6502 matching lines @@
               },
               "localizedName": {
                 "type": "string",
                 "optional": true,
                 "description": "A string to use instead of the raw value of the 'name' key from manifest.json."
               },
               "locale": {
                 "type": "string",
                 "optional": true,
                 "description": "The name of the locale used for generating localizedName. This should be the name of one of the directories in the _locales folder of the extension, or the default_locale setting from the manifest."
+              },
+              "appInstallBubble": {
+                "type": "boolean",
+                "optional": true,
+                "description": "A flag to change the UI we show when an app is installed - a value of true means to show a bubble pointing at the new tab button (instead of the default behavior of opening the new tab page and animating the app icon)."
               }
             }
           },
           {
             "name": "callback",
             "type": "function",
             "description": "Called when the user has either accepted/rejected the dialog, or some error occurred (such as invalid manifest or icon image data).",
             "optional": "true",
             "parameters": [
               {
@@ skipping 815 matching lines @@
                 "description": "A string result code. The value is non-empty on success only in tests.",
                 "optional": "true"
               }
             ]
           }
         ]
       }
     ]
   }
 ]