Remove dependency on headers stream from QuicSession.

Remove dependency on headers stream from QuicSession.

Previously QuicSession was always creating headers stream with
stream_id=3. That stream is useless for P2P QUIC connections. Added
new QuicSpdySession class and moved all headers-related logic from
QuicSession to QuicSpdySession.

relnote: QUIC refactoring, doesn't affect functionality

Merge internal change: 95762401

R=rch@chromium.org, sergeyu@chromium.org

Committed: https://crrev.com/b865eb88a8e8834ca0f5c6e9e55288bf43e5d92b
Cr-Commit-Position: refs/heads/master@{#334903}

[ramant (doing other things), 2015-06-17 00:58:43]

Hi Ryan,
  This CL required little bit of porting (or changes) to chromium specific
classes. Would like to land this CL individually. PTAL.

thanks
raman

[Ryan Hamilton, 2015-06-17 13:49:10]

On 2015/06/17 00:58:43, ramant wrote:
> Hi Ryan,
>   This CL required little bit of porting (or changes) to chromium specific
> classes. Would like to land this CL individually. PTAL.
> 
> thanks
> raman

Nice job. LGTM!

[ramant (doing other things), 2015-06-17 16:58:23]

The CQ bit was checked by rtenneti@chromium.org

[commit-bot: I haz the power, 2015-06-17 16:58:38]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1190823003/20001

[commit-bot: I haz the power, 2015-06-17 18:41:27]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-17 18:41:28]

Try jobs failed on following builders:
  linux_android_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_android_r...)

[ramant (doing other things), 2015-06-17 19:23:16]

The CQ bit was checked by rtenneti@chromium.org

[commit-bot: I haz the power, 2015-06-17 19:24:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1190823003/20001

[commit-bot: I haz the power, 2015-06-17 20:21:54]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-17 20:23:00]

Patchset 2 (id:??) landed as
https://crrev.com/b865eb88a8e8834ca0f5c6e9e55288bf43e5d92b
Cr-Commit-Position: refs/heads/master@{#334903}