drive: Split DirectoryLoader from ChangeListLoader

Directory loading related code is moved to DirectoryLoader.
ChangeListLoader::LoadDirectoryIfNeeded is implemented with DirectoryLoader.

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

Review URL: https://codereview.chromium.org/174073011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252894 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252894