Added unit test for new portion of GetMetaInfo API

BUG=383600

Review URL: https://codereview.chromium.org/348833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278908 0039d316-1c4b-4281-b951-d872f2087c98
   ExtensionBookmarksTest()

   ExtensionBookmarksTest()
      : client_(NULL), model_(NULL), node_(NULL), folder_(NULL) {}
