ServiceWorker: Add constructor(sequence<sequence<ByteString>>) to Headers

The IDL compiler now supports nested sequence for arguments. Let's use it
instead of Dictionary. We still need a constructor that takes Dictionary
because we don't support OpenEndedDictionary yet.

Tests under http/tests/serviceworker cover this change.

Review URL: https://codereview.chromium.org/613953002

git-svn-id: svn://svn.chromium.org/blink/trunk@183016 bbb929c8-8fbe-4397-9dbb-9b2b20218538