Fix QuicHttpStream reporting negative sent bytes.

It was treating error codes as number of header bytes sent.
This may have affected metrics slightly, not sure if it had any
had any web-visible ordevtools-visible effects.

BUG=731819

Review-Url: https://codereview.chromium.org/2937113002
Cr-Commit-Position: refs/heads/master@{#479749}
2 files changed