Remove parent* arg from GDataEntry ctor.

* Remove static FromDocumentEntry from GDataEntry, GDataFile, GDataDirectory. Replace with InitFromDocumentEntry.
* Move common code from GDataFile::InitFromDocumentEntry and GDataDirectory::InitFromDocumentEntry to GDataEntry::InitFromDocumentEntry.
* Add GDataDirectoryService::FromDocumentEntry and use this everywhere.
* Make ctors of GDataFile, GDataDirectory private, so these must be created by GDataDirectoryService's CreateGDataFile and
CreateGDataDirectory. Make GDataEntry ctor protected.

BUG=141494
TEST=unit tests.
Review URL: https://chromiumcodereview.appspot.com/10854083

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151008 0039d316-1c4b-4281-b951-d872f2087c98
void GDataDirectoryService::SerializeToString(
    std::string* serialized_proto) const {
  GDataRootDirectoryProto proto;
  root_->ToProto(proto.mutable_gdata_directory());
  proto.set_largest_changestamp(largest_changestamp_);
  proto.set_version(kProtoVersion);

  const bool ok = proto.SerializeToString(serialized_proto);
  DCHECK(ok);
}

void GDataDirectoryService::SerializeToString(
    std::string* serialized_proto) const {
  GDataRootDirectoryProto proto;
  root_->ToProto(proto.mutable_gdata_directory());
  proto.set_largest_changestamp(largest_changestamp_);
  proto.set_version(kProtoVersion);

  const bool ok = proto.SerializeToString(serialized_proto);
  DCHECK(ok);
}
