Put more files under the enable_service_discovery build flag.

chrome/browser/extensions/BUILD.gn

Index: chrome/browser/extensions/BUILD.gn
diff --git a/chrome/browser/extensions/BUILD.gn b/chrome/browser/extensions/BUILD.gn
index 46242cbc8c9f05b42c5b744401c4376eb555c3ac..e6d00d95779efead2675b35fd380c12adad44787 100644
--- a/chrome/browser/extensions/BUILD.gn
+++ b/chrome/browser/extensions/BUILD.gn
@@ -111,6 +111,13 @@ source_set("extensions") {
     }
   }
 
+  if (enable_service_discovery) {
+    sources += rebase_path(
+            gypi_values.chrome_browser_extensions_service_discovery_sources,
+            ".",
+            "//chrome")
+  }
+
   if (use_ash) {
     sources += [
       "api/tabs/ash_panel_contents.cc",