Add constructor for DataTransfer

This is being added to support the Asynchronous Clipboard API, which
requires a DataTransfer object to store the data for the write() operation.

DataTransfer:
Intent to Implement & Ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/NY8OPSzHGuE
WHATWG HTML spec bug: https://github.com/whatwg/html/issues/2190
Spec: https://html.spec.whatwg.org/multipage/interaction.html#the-datatransfer-interface
W3C WebPlatformTest PR: https://github.com/w3c/web-platform-tests/pull/5232

Async Clipboard:
W3C Spec: https://w3c.github.io/clipboard-apis/#async-clipboard-api

BUG=677564

Review-Url: https://codereview.chromium.org/2784473003
Cr-Commit-Position: refs/heads/master@{#461281}
6 files changed