gdata: Define the resource ID for the root directory

Per the spec, the resource ID for the root directory is defined
as "folder:root". Add the resource ID to the root directory in our
file system representation so we can look up the root directory by
the resource ID.

BUG=127697
TEST=add unit tests

Review URL: https://chromiumcodereview.appspot.com/10332253

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137928 0039d316-1c4b-4281-b951-d872f2087c98
 std::string CacheSubDirectoryTypeToString(
     GDataRootDirectory::CacheSubDirectoryType subdir) {
  switch (subdir) {
    case GDataRootDirectory::CACHE_TYPE_META: return "meta";
    case GDataRootDirectory::CACHE_TYPE_PINNED: return "pinned";
    case GDataRootDirectory::CACHE_TYPE_OUTGOING: return "outgoing";
    case GDataRootDirectory::CACHE_TYPE_PERSISTENT: return "persistent";
    case GDataRootDirectory::CACHE_TYPE_TMP: return "tmp";
    case GDataRootDirectory::CACHE_TYPE_TMP_DOWNLOADS:
      return "tmp_downloads";
    case GDataRootDirectory::CACHE_TYPE_TMP_DOCUMENTS:
      return "tmp_documents";
    case GDataRootDirectory::NUM_CACHE_TYPES:
      NOTREACHED();
  }
  NOTREACHED();
  return "unknown subdir";
}

 std::string CacheSubDirectoryTypeToString(
     GDataRootDirectory::CacheSubDirectoryType subdir) {
  switch (subdir) {
    case GDataRootDirectory::CACHE_TYPE_META: return "meta";
    case GDataRootDirectory::CACHE_TYPE_PINNED: return "pinned";
    case GDataRootDirectory::CACHE_TYPE_OUTGOING: return "outgoing";
    case GDataRootDirectory::CACHE_TYPE_PERSISTENT: return "persistent";
    case GDataRootDirectory::CACHE_TYPE_TMP: return "tmp";
    case GDataRootDirectory::CACHE_TYPE_TMP_DOWNLOADS:
      return "tmp_downloads";
    case GDataRootDirectory::CACHE_TYPE_TMP_DOCUMENTS:
      return "tmp_documents";
    case GDataRootDirectory::NUM_CACHE_TYPES:
      NOTREACHED();
  }
  NOTREACHED();
  return "unknown subdir";
}
