Media Galleries API Scanning: Add Scan Permission and basic test.

chrome/common/extensions/api/_permission_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.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
 //
 // $ echo -n "aaaabbbbccccddddeeeeffffgggghhhh" | \
@@ skipping 501 matching lines @@
     }
   ],
   "mediaGalleries": {
     "channel": "stable",
     "extension_types": [ "platform_app" ]
   },
   "mediaGalleries.allAutoDetected": {
     "channel": "stable",
     "extension_types": [ "platform_app" ]
   },
+  "mediaGalleries.scan": {
+    "channel": "dev",
+    "extension_types": [ "platform_app" ]
+  },
   "mediaGalleries.read": {
     "channel": "stable",
     "extension_types": [ "platform_app" ]
   },
   "mediaGalleries.copyTo": {
     "channel": "stable",
     "extension_types": [ "platform_app" ]
   },
   "mediaGalleries.delete": {
     "channel": "stable",
@@ skipping 405 matching lines @@
     // Only allowed for whitelisted extensions until all the caveats are
     // addressed. Tracked in crbug/285151.
     "channel": "stable",
     "extension_types": ["extension"],
     "location": "component",
     "whitelist": [
       "D519188F86D9ACCEE0412007B227D9936EB9676B"  // GAIA Component Extension
     ]
   }]
 }