google_apis: Implement google_apis::ResponseWriter

Remove URLFetcherDelegate::OnURLFetchDownloadData.
Stop resetting response_writer_ in URLFetcherCore::CancelURLRequest() to guarantee that the writer object is always alive if URLFetcher is alive.
To close the file, call URLFetcherResponseWriter::Finish when URLFetcherCore::URLFetcherCore::WriteBuffer fails.
google_apis::ResponseWriter is responsible to call GetContentCallback.

BUG=126753
TEST=unit_tests

Review URL: https://codereview.chromium.org/26784003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230992 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230992