Allow chrome.systemPrivate to be used by apps.

BUG=367364

Review URL: https://codereview.chromium.org/423543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285799 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/_permission_features.json

@@ -968,7 +968,7 @@
   },
   "systemPrivate": {
     "channel": "stable",
-    "extension_types": ["extension", "legacy_packaged_app"],
+    "extension_types": ["extension", "legacy_packaged_app", "platform_app"],
     "location": "component"
   },
   "tabs": [