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
void LocalReaderProxy::OnCompleted(FileError error) {
  DCHECK_EQ(FILE_ERROR_OK, error);
 }

void LocalReaderProxy::OnCompleted(FileError error) {
  DCHECK_EQ(FILE_ERROR_OK, error);
 }
