Updated DOMString type with USVString for window.open url
to match the html specification.

https://html.spec.whatwg.org/multipage/window-object.html#the-window-object

Bug: 790860
Change-Id: I371655167c3a99c121512cad91be950c75fc0a93
Reviewed-on: https://chromium-review.googlesource.com/829153
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Yuki Shiino <[email protected]>
Commit-Queue: Bhagirathi Satpathy <[email protected]>
Cr-Commit-Position: refs/heads/master@{#525014}