[Explore Sites] Adding URL fetcher for Explore Sites.

This will be used to fetch the data from our API endpoint. It is only a fetcher
and does not do any processing of the data that is downloaded.

Bug: 867488
Change-Id: Ic7506af2523632c1173333ddf2a1f2183b27f72f
Reviewed-on: https://chromium-review.googlesource.com/1229013
Commit-Queue: Jonathan Freed <freedjm@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592919}
12 files changed