Fix AsyncFileTestHelper::ReadDirectory to handle has_more correctly

BUG=347900
TEST=content_unittests

Review URL: https://codereview.chromium.org/181063031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255592