Use url.mojom.Url in ProvisionFetcher mojo interface.

Bug: 662752
Change-Id: I9524e40e3645f11841de4805980d905a5ac25143
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500762


Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Alex St-Onge <alstonge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823259}