Land Recent QUIC changes.

Land Recent QUIC changes.

Add another empty packet check to QuicPacketCreator.

Merge internal change: 51128171

Change a DCHECK into LOG(DFATAL) so that we can see if this is actually
happening.

Merge internal change: 51096918

Remove hard-coded version from QuicFramerTest

Merge internal change: 51006687

Reserializing packets for retransmission now use the original sequence
number length, as intended, and a test has been added to verify that.

Also, re-calculating the packet's size anytime no frames have been
added.  Once they've been added, the header size will not change, as
before.

Merge internal change: 51153356

Lazily initialize QuicPacketCreator's packet_size to the header size,
so the sequence_number_length and other overhead can be more accurately
calculated.

Merge internal change: 51071397

Enable variable length sequence numbers in the packet header by
dynamically calculating the necessary length.

Merge internal change: 50917379

R=rch@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23368002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@218781 0039d316-1c4b-4281-b951-d872f2087c98