Refactoring: Create per-connection packet writers in QuicDispatcher.

Refactoring: Create per-connection packet writers in QuicDispatcher.

To make porting the QUIC EndToEndTest to Chromium possible with fewer
Chromium-specific parts in shared code, I've made QuicDispatcher expose
and accept a QuicDispatcher::PacketWriterFactory which it uses to create
a new packet writer wrapper for every QuicConnection. I also changed
QuicConnection to accept a QuicConnection::PacketWriterFactory (a second
new type of factory) rather than the writer itself in its constructor,
since the per-connection packet writers need to be created with the
connection already existing.

Merge internal CL: 73064412

Written by Daniel Ziegler <dmziegler@chromium.org>;
Original review URL: https://codereview.chromium.org/467963002/

R=rch@chromium.org,wtc@chromium.org
BUG=

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291314

[wtc, 2014-08-21 01:07:46]

Ryan,

This is a clone of Daniel's CL.

Patch set 1 is essentially the original internal CL.

Patch set 2 is Daniel's port of the internal CL.

Patch set 3 includes additional cleanups made by me.

Please review this CL. If Daniel doesn't have time to rebase and commit his CL
this week, I'll commit this CL.

[Ryan Hamilton, 2014-08-21 22:16:22]

lgtm

[wtc, 2014-08-21 23:56:11]

The CQ bit was checked by wtc@chromium.org

[commit-bot: I haz the power, 2014-08-21 23:59:35]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/wtc@chromium.org/475113005/80001

[commit-bot: I haz the power, 2014-08-22 01:58:11]

Committed patchset #5 (80001) as 291314