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/extensions/BUILD.gn b/chrome/browser/extensions/BUILD.gn
index beb0f01..537af73c 100644
--- a/chrome/browser/extensions/BUILD.gn
+++ b/chrome/browser/extensions/BUILD.gn
@@ -99,7 +99,9 @@
                     ".",
                     "//chrome")
     sources -= [ "api/music_manager_private/device_id_linux.cc" ]
-    configs += [ "//build/config/linux/dbus" ]
+    if (use_dbus) {
+      configs += [ "//build/config/linux/dbus" ]
+    }
     deps += [
       "//components/chrome_apps",
       "//remoting/host/it2me:common",
@@ -143,10 +145,12 @@
   }
 
   if (is_linux) {
-    deps += [
-      "//build/linux:fontconfig",
-      "//dbus",
-    ]
+    deps += [ "//build/linux:fontconfig" ]
+
+    if (use_dbus) {
+      deps += [ "//dbus" ]
+    }
+
     if (use_x11) {
       configs += [ "//build/config/linux:x11" ]
       deps += [