[Presentation API] Add support for multiple URLs in PresentationRequest on Media Router UI side

chrome/browser/media/router/presentation_media_sinks_observer.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/media/router/presentation_media_sinks_observer.h"
 
 #include "chrome/browser/media/router/media_router.h"
 #include "chrome/browser/media/router/media_source.h"
 #include "content/public/browser/presentation_screen_availability_listener.h"
 
 namespace media_router {
 
 PresentationMediaSinksObserver::PresentationMediaSinksObserver(
     MediaRouter* router,
     content::PresentationScreenAvailabilityListener* listener,
     const MediaSource& source,
     const GURL& origin)
     : MediaSinksObserver(router, source, origin),
       listener_(listener),
       previous_availablity_(UNKNOWN) {
   DCHECK(router);
   DCHECK(listener_);
-  DCHECK(!source.Empty());
 }
 
 PresentationMediaSinksObserver::~PresentationMediaSinksObserver() {
 }
 
 void PresentationMediaSinksObserver::OnSinksReceived(
     const std::vector<MediaSink>& result) {
   Availability current_availability = result.empty() ? UNAVAILABLE : AVAILABLE;
 
   DVLOG(1) << "PresentationMediaSinksObserver::OnSinksReceived: "
            << source().ToString() << " "
            << (result.empty() ? "unavailable" : "available");
 
   // Don't send if new result is same as previous.
   if (previous_availablity_ != current_availability)
     listener_->OnScreenAvailabilityChanged(current_availability == AVAILABLE);
 
   previous_availablity_ = current_availability;
 }
 
 }  // namespace media_router