Fix Presentation API mocks: do not eagerly drop clients.

content/shell/renderer/test_runner/mock_presentation_service.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 "content/shell/renderer/test_runner/mock_presentation_service.h"
 
 #include "base/logging.h"
 
 namespace content {
 
@@ skipping 11 matching lines @@
   screen_availability_ = available;
 
   FOR_EACH_OBSERVER(
       MockPresentationClient,
       presentation_clients_,
       SetScreenAvailability(available));
 }
 
 void MockPresentationService::Reset() {
   FOR_EACH_OBSERVER(MockPresentationClient, presentation_clients_, Reset());
-  presentation_clients_.Clear();
   screen_availability_ = false;
 }
 
 void MockPresentationService::RegisterPresentationClientMock(
     MockPresentationClient* client) {
   DCHECK(client);
   presentation_clients_.AddObserver(client);
 }
 
 void MockPresentationService::UnregisterPresentationClientMock(
     MockPresentationClient* client) {
   DCHECK(client);
   presentation_clients_.RemoveObserver(client);
 }
 
 }  // namespace content