[Media Router] Conditionally enable mDNS on Windows.

chrome/browser/media/router/media_router_mojo_impl.h

Index: chrome/browser/media/router/media_router_mojo_impl.h
diff --git a/chrome/browser/media/router/media_router_mojo_impl.h b/chrome/browser/media/router/media_router_mojo_impl.h
index 35ca51e2c872408614870b106f10dcdf9677ca10..4927838b5319e591641126830414aad019baa7d0 100644
--- a/chrome/browser/media/router/media_router_mojo_impl.h
+++ b/chrome/browser/media/router/media_router_mojo_impl.h
@@ -98,6 +98,8 @@ class MediaRouterMojoImpl : public MediaRouterBase,
       const SendRouteMessageCallback& callback) override;
   void AddIssue(const Issue& issue) override;
   void ClearIssue(const Issue::Id& issue_id) override;
+  bool should_enable_mdns_discovery() const override;
+  void EnableMdnsDiscovery() override;
 
   const std::string& media_route_provider_extension_id() const {
     return media_route_provider_extension_id_;
@@ -381,6 +383,12 @@ class MediaRouterMojoImpl : public MediaRouterBase,
   // initial event page wakeup attempt.
   bool provider_version_was_recorded_ = false;
 
+  // A flag to ensure that mDNS discovery is only enabled on Windows when there
+  // will be appropriate context for the user to associate a firewall prompt
+  // with Media Router. This can be set to |true| earlier if we know that a
+  // prompt will not occur.
+  bool should_enable_mdns_discovery_ = true;
+
   base::WeakPtrFactory<MediaRouterMojoImpl> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaRouterMojoImpl);