Configure QUIC max packet size via a Field Trial or command line arg.
When configured via field trial we well extract the packet size from the name of the field trial group.

Review URL: https://codereview.chromium.org/84273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237481 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237481