Only activate DnsSdDeviceLister::Discover() when possible.

If the ENABLE_SERVICE_DISCOVERY build flag is not set, then this code
will never work correctly, because DnsSdRegistry will not pass in a
valid ServiceDiscoverySharedClient. So just make Discover() a no-op in
this case, guarded by ENABLE_SERVICE_DISCOVERY. This in turn, makes some
DnsSdDeviceLister member variables only used when
ENABLE_SERVICE_DISCOVERY is set, so guard them by the same build flag.

Bug: 757530
Change-Id: I2f0864e1342be3d6328fbf449dc2296825fe1fbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716102Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680473}

chrome/browser/media/router/discovery/mdns/dns_sd_device_lister.cc

@@ -32,13 +32,19 @@ DnsSdDeviceLister::DnsSdDeviceLister(
     local_discovery::ServiceDiscoveryClient* service_discovery_client,
     DnsSdDelegate* delegate,
     const std::string& service_type)
-    : delegate_(delegate),
+    : delegate_(delegate)
+#if BUILDFLAG(ENABLE_SERVICE_DISCOVERY)
+      ,
       service_discovery_client_(service_discovery_client),
-      service_type_(service_type) {}
+      service_type_(service_type)
+#endif
+{
+}
 
 DnsSdDeviceLister::~DnsSdDeviceLister() {}
 
 void DnsSdDeviceLister::Discover() {
+#if BUILDFLAG(ENABLE_SERVICE_DISCOVERY)
   if (!device_lister_) {
     device_lister_ = local_discovery::ServiceDiscoveryDeviceLister::Create(
         this, service_discovery_client_, service_type_);
@@ -49,6 +55,7 @@ void DnsSdDeviceLister::Discover() {
   device_lister_->DiscoverNewDevices();
   VLOG(1) << "Discovery new devices for service type "
           << device_lister_->service_type();
+#endif
 }
 
 void DnsSdDeviceLister::Reset() {

chrome/browser/media/router/discovery/mdns/dns_sd_device_lister.h

@@ -10,6 +10,7 @@
 
 #include "base/macros.h"
 #include "chrome/browser/local_discovery/service_discovery_device_lister.h"
+#include "chrome/common/buildflags.h"
 
 namespace local_discovery {
 class ServiceDiscoveryClient;
@@ -49,12 +50,14 @@ class DnsSdDeviceLister
   // The delegate to notify of changes to services.
   DnsSdDelegate* const delegate_;
 
-  // Created when |Discover()| is called.
+  // Created when Discover() is called, if service discovery is enabled.
   std::unique_ptr<local_discovery::ServiceDiscoveryDeviceLister> device_lister_;
 
+#if BUILDFLAG(ENABLE_SERVICE_DISCOVERY)
   // The client and service type used to create |device_lister_|.
   local_discovery::ServiceDiscoveryClient* const service_discovery_client_;
   const std::string service_type_;
+#endif
 
   DISALLOW_COPY_AND_ASSIGN(DnsSdDeviceLister);
 };