Support MessageEventInit.ports sequences.

For MessageEvent constructor dictionaries,

 http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#messageevent

don't limit ourselves to array objects, but support the required
sequence type.

R=
BUG=

Review URL: https://codereview.chromium.org/145323005

git-svn-id: svn://svn.chromium.org/blink/trunk@165656 bbb929c8-8fbe-4397-9dbb-9b2b20218538