Update Crostini image server URL

The image server URL should have a %d in it, to be replaced with
the Chrome OS milestone at runtime.

BUG=chromium:918970
TEST=set up crostini; check that container has wget preinstalled

Change-Id: I3cdfce9ea1b80e704719da05449734c82d2b7dc9
Reviewed-on: https://chromium-review.googlesource.com/c/1395025Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Stephen Barber <smbarber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619840}

chrome/browser/chromeos/crostini/crostini_util.h

@@ -126,7 +126,7 @@ constexpr char kCrostiniCroshBuiltinAppId[] =
 // Generated using crx_file::id_util::GenerateId("LinuxAppsFolder")
 constexpr char kCrostiniFolderId[] = "ddolnhmblagmcagkedkbfejapapdimlk";
 constexpr char kCrostiniDefaultImageServerUrl[] =
-    "https://storage.googleapis.com/cros-containers";
+    "https://storage.googleapis.com/cros-containers/%d";
 constexpr char kCrostiniDefaultImageAlias[] = "debian/stretch";
 
 // Whether running Crostini is allowed for unaffiliated users per enterprise