[SystemInfo API] Implement storage.getAvailableCapacity API

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // See c/c/e/features/* to understand this file, in particular feature.h,
 // simple_feature.h, and base_feature_provider.h.
 
 {
   "activityLogPrivate": {
     "dependencies": ["permission:activityLogPrivate"],
@@ skipping 549 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "system.memory": {
     "dependencies": ["permission:system.memory"],
     "contexts": ["blessed_extension"]
   },
   "system.storage": {
     "dependencies": ["permission:system.storage"],
     "contexts": ["blessed_extension"]
   },
+  "system.storage.getAvailableCapacity": {
+    "channel": "dev"
+  },
   "systemPrivate": {
     "dependencies": ["permission:systemPrivate"],
     "contexts": ["blessed_extension"]
   },
   "tabCapture": {
     "dependencies": ["permission:tabCapture"],
     "contexts": ["blessed_extension"]
   },
   "tabs": {
     "channel": "stable",
@@ skipping 97 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }