Implement QUIC's FLAGS_fake_packet_loss_percentage in chrome.

Review URL: https://chromiumcodereview.appspot.com/24276009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225001 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225001