Remove suppressions for MessagePortChannel

These were fixed four years after they were filed, in
http://src.chromium.org/viewvc/blink?view=rev&rev=161083

BUG=27838,28200,311229
TBR=timurrrr@chromium.org

Review URL: https://codereview.chromium.org/69953027

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235258 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235258