Crostini: Allow an alternate crostini image server url to be specified

This will be used for loading specific container images during automated
testing.  This allows the images to be loaded using the same code paths
in lxd and other system components rather than the current approach
which is quite different from the way that users actually install
Crostini.

Design proposal at https://docs.google.com/document/d/1-T0tpU9szQxXVHtK_qXXG6JYjiQazQAQ28sIz8WNt_Q/edit#heading=h.5irk4csrpu0y

Bug: None
Change-Id: Ib4753fd1e4fd33b3207f0624e1325ffc7a742b7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2135095Reviewed-by: Nic Hollingum <hollingum@google.com>
Commit-Queue: Matthew Warton <mwarton@google.com>
Cr-Commit-Position: refs/heads/master@{#756555}

chrome/browser/chromeos/crostini/crostini_manager.cc

@@ -1352,7 +1352,17 @@ void CrostiniManager::CreateLxdContainer(std::string vm_name,
   request.set_vm_name(std::move(vm_name));
   request.set_container_name(std::move(container_name));
   request.set_owner_id(owner_id_);
-  request.set_image_server(kCrostiniDefaultImageServerUrl);
+  std::string image_server_url;
+  auto* cros_component_manager =
+      g_browser_process->platform_part()->cros_component_manager();
+  if (cros_component_manager) {
+    image_server_url = cros_component_manager
+                           ->GetCompatiblePath("cros-crostini-image-server-url")
+                           .value();
+  }
+  request.set_image_server(image_server_url.empty()
+                               ? kCrostiniDefaultImageServerUrl
+                               : image_server_url);
   if (base::FeatureList::IsEnabled(
           chromeos::features::kCrostiniUseBusterImage)) {
     request.set_image_alias(kCrostiniBusterImageAlias);