problem description
when I am working on the chrome extension, I need to use the api, of fileSystem to add the fileSystem permission to permissions, but at run time, chrome.fileSystem is still undefined, which is the configuration of permissions
    "permissions": [
        "contextMenus",
        "tabs",
        "webRequest",
        "webRequestBlocking",
        "storage",
        "system.display",
        "fileSystem"
    ],Why is this?
later I used the native fileSystem api, of html5 because I wanted to persist the data and could not be cleared by the operation of clearing the browser cache, so I wrote the following code in the background.js of the plug-in,
    window.requestFileSystem = window.requestFileSystem || window.webkitRequestFileSystem;
    navigator.webkitPersistentStorage.requestQuota(bytes, function (grantedBytes) {
        alert(":" + grantedBytes);
        window.requestFileSystem(window.PERSISTENT, grantedBytes, initFs, errorHandler);
    }, errorHandler); as a result, the grantedBytes value in the callback function is 0. 
 does it mean that plug-ins can"t apply for presistent space? 
