Fix OOB Write in QuicStreamSequencerBuffer::OnStreamData

BUG=778505

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I1dfd1d26a2c7ee8fe047f7fe6e4ac2e9b97efa52
Reviewed-on: https://chromium-review.googlesource.com/748282
Commit-Queue: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513144}
QuicStreamOffset QuicStreamSequencerBuffer::BytesConsumed() const {
  return total_bytes_read_;
}

QuicStreamOffset QuicStreamSequencerBuffer::BytesConsumed() const {
  return total_bytes_read_;
}
