[Fetch API] Have RequestInit's member variables private

I would like to introduce methods to it and hence I want it to be a
regular class.

Bug: 695939
Change-Id: Ie073bc753d61f18a210ef1863a6b91f24342e7c2
Reviewed-on: https://chromium-review.googlesource.com/720860
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509037}
3 files changed