drive: No dependency between ChangeListLoader and DirectoryLoader

Move LoadDirectoryIfNeeded from ChangeListLoader to FileSystem
Remove ChangeListLoader::LoadForTesting().
Add DirectoryLoaderTest.

BUG=335469
TEST=unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/179323002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253343 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253343