Handle webuis when using the network service.
Some notes:
-once PlzNavigate and Mojo loading ship, then we can use this code path in production. at that point, URLDataManagerBackend should move to the UI thread which would avoid thread hops
-NavigationURLLoaderNetworkService knows about this scheme, but that should be abstracted out later to support other schemes
BUG=717714
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation
Review-Url: https://codereview.chromium.org/2860903006
Cr-Commit-Position: refs/heads/master@{#469848}
16 files changed
