The Cast Linux build will use this interface to provision devices
from the "media" service. Move these classes into general subdirs and
update BUILD.gn files so that these build on Linux.

The only non-mechanical change in this CL is that
content::CreateProvisionFetcher() now takes a net::URLRequestContextGetter*
instead of a RenderFrameHost*. This will allow more flexibility for content
embedders like Chromecast to use this class.

BUG=660736
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2469863010
Cr-Commit-Position: refs/heads/master@{#430308}