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 GDataEntry::UnescapeUtf8FileName(const std::string& input) {
  std::string output = input;
  ReplaceSubstringsAfterOffset(&output, 0, std::string(kEscapedSlash), kSlash);
  return output;
}

std::string GDataEntry::UnescapeUtf8FileName(const std::string& input) {
  std::string output = input;
  ReplaceSubstringsAfterOffset(&output, 0, std::string(kEscapedSlash), kSlash);
  return output;
}
