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}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index c4e6c16e..a107c23 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -327,7 +327,7 @@
       FrameTreeNode::GloballyFindByID(frame_tree_node_id);
   if (!frame_tree_node)
     return nullptr;
-  return FromRenderFrameHost(frame_tree_node->current_frame_host());
+  return WebContentsImpl::FromFrameTreeNode(frame_tree_node);
 }
 
 void WebContents::SetScreenOrientationDelegate(