Implement range reading for NetworkReaderProxy.

The feature is not yet actually used, but will be used for range reading
of DriveFileStreamReader.

BUG=127129
TEST=Ran unit_tests

Review URL: https://chromiumcodereview.appspot.com/14493008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@196636 0039d316-1c4b-4281-b951-d872f2087c98
int DriveFileStreamReader::Read(net::IOBuffer* buffer, int buffer_length,
                                const net::CompletionCallback& callback) {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
  DCHECK(reader_proxy_);
  DCHECK(buffer);
  DCHECK(!callback.is_null());
  return reader_proxy_->Read(buffer, buffer_length, callback);
}

int DriveFileStreamReader::Read(net::IOBuffer* buffer, int buffer_length,
                                const net::CompletionCallback& callback) {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
  DCHECK(reader_proxy_);
  DCHECK(buffer);
  DCHECK(!callback.is_null());
  return reader_proxy_->Read(buffer, buffer_length, callback);
}
