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=

Review URL: https://codereview.chromium.org/475113005

Cr-Commit-Position: refs/heads/master@{#291314}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291314 0039d316-1c4b-4281-b951-d872f2087c98