Land Recent QUIC changes.

Enables the padding of QUIC handshake packets to the max packet
size. Does not pad packets which do not leave enough space to fit a
padding frame after expanding the final stream frame.  An alternative
would be to add a new type of padding frame which could show up
earlier in the packet, but this is good enough for now.

     flag to enable the padding of QUIC handshake packets to full size

Merge internal change: 55118010

Add a set_seed method to the PacketDroppingTestWriter to facilitate
reproducing test behavior.

Merge internal change: 55107501

Consistently pass the IsHandshake flag to the congestion control
algorithm in QUIC.

Merge internal change: 55046282

The goal is parity between HTTP and QUIC testing.

Merge internal change: 55000555

Fixed comments - found while back porting changes to server.

Merge internal change: 54999641

Fix bug in QuicPacketCreator's packet size calculation when a stream
frame is close to the maximum possible.

Merge internal change: 54933729

Remove unused random_reorder field from QuicPacketCreator::Options

Merge internal change: 54885136

Using GFE's IOVector to replace uses of struct iovec on the QUIC's
Write path.

Merge internal change: 54805402

R=rch@chromium.org

Review URL: https://codereview.chromium.org/46903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231278 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231278