message: Fix GCS filesystem on windows by using platform independent primitives in GetMatchingPaths
  This makes all GcsfileSystem tests pass on windows.
diff: @@ -1331,7 +1331,7 @@ Status GcsFileSystem::GetMatchingPaths(const string& pattern,
  // Find the fixed prefix by looking for the first wildcard.
  const string& fixed_prefix =
  pattern.substr(0, pattern.find_first_of("*?[\\"));
  - const string dir(io::Dirname(fixed_prefix));
  + const string dir(this->Dirname(fixed_prefix));
  if (dir.empty()) {
  return errors::InvalidArgument(
  "A GCS pattern doesn't have a bucket name: ", pattern);
  @@ -1345,8 +1345,8 @@ Status GcsFileSystem::GetMatchingPaths(const string& pattern,
  // Match all obtained paths to the input pattern.
  for (const auto& path : files_and_folders) {
  - const string& full_path = io::JoinPath(dir, path);
  - if (Env::Default()->MatchPath(full_path, pattern)) {
  + const string& full_path = this->JoinPath(dir, path);
  + if (this->Match(full_path, pattern)) {
  results->push_back(full_path);
  }
  }
  