Guard dbus component and config with assert(use_dbus)

Often times dbus dependencies get added without proper guards.
By guarding the component with assert(use_dbus) it forces proper guarding
when depending on dbus. This prevents downstream breakages in
chromecast.

BUG=internal b/28962923

Review-Url: https://codereview.chromium.org/2009983002
Cr-Commit-Position: refs/heads/master@{#396948}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 5df14a4b..dfc340a 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -867,11 +867,10 @@
 
   if (is_linux) {
     if (use_aura) {
-      configs += [ "//build/config/linux/dbus" ]
-      deps += [
-        "//build/linux:fontconfig",
-        "//dbus",
-      ]
+      deps += [ "//build/linux:fontconfig" ]
+      if (use_dbus) {
+        deps += [ "//dbus" ]
+      }
     }
     if (use_x11) {
       configs += [ "//build/config/linux:x11" ]