Simplify MessageChannel's IDL

Simplify MessageChannel's IDL by removing useless Constructor-related
extended attributes. Those have no effect since we use custom code for
the constructor.

R=haraken

Review URL: https://chromiumcodereview.appspot.com/23055004

git-svn-id: svn://svn.chromium.org/blink/trunk@156289 bbb929c8-8fbe-4397-9dbb-9b2b20218538