Fix bug in QuicHttpStream::SendRequest.

Fix bug in QuicHttpStream::SendRequest.
If the session is closed before SendRequest is called, a CHECK would fire in SendRequest().

BUG=409101
Committed: https://crrev.com/f9f103cbc943019e6576a68c724a6360da26b687
Cr-Commit-Position: refs/heads/master@{#292775}

[Ryan Hamilton, 2014-08-29 21:43:32]

rch@chromium.org changed reviewers:
+ rtenneti@chromium.org

[ramant (doing other things), 2014-08-29 22:32:06]

lgtm

[Ryan Hamilton, 2014-08-30 03:43:26]

The CQ bit was checked by rch@chromium.org

[commit-bot: I haz the power, 2014-08-30 03:43:57]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rch@chromium.org/523463007/1

[commit-bot: I haz the power, 2014-08-30 05:29:00]

Committed patchset #1 (id:1) as a9fc109a109b1c420dca1d054ee63427a07a6166

[commit-bot: I haz the power, 2014-09-10 03:13:50]

Patchset 1 (id:??) landed as
https://crrev.com/f9f103cbc943019e6576a68c724a6360da26b687
Cr-Commit-Position: refs/heads/master@{#292775}