disconnect() is now called on the event window.onbeforeunload instead of
window.onunload to ensure we properly disconnect from
peerconnection_server

BUG=302858
TEST=MANUAL disconnecting, closing tab and chrome and it disconnects properly 
from peerconnection_server
NOTRY=TRUE

Review URL: https://codereview.chromium.org/27534002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228925 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228925