Construct URLSearchParams from sequence initializer.

Follow up recent spec addition[1,2] and support sequence<sequence<USVString>>
initializers for URLSearchParams.

1 - https://github.com/whatwg/url/issues/27
2 - https://github.com/whatwg/url/pull/175

R=tyoshino,haraken
BUG=680531

Review-Url: https://codereview.chromium.org/2725593003
Cr-Commit-Position: refs/heads/master@{#453903}
6 files changed