Fix Presentation API mocks: do not eagerly drop clients.

content/shell/renderer/test_runner/mock_presentation_client.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_client.h"
 
 #include "base/logging.h"
 #include "content/shell/renderer/test_runner/mock_presentation_service.h"
 #include "third_party/WebKit/public/platform/modules/presentation/WebPresentationController.h"
 
 namespace content {
 
 MockPresentationClient::MockPresentationClient(MockPresentationService* service)
     : controller_(nullptr),
       screen_availability_(false),
       service_(service) {
   DCHECK(service_);
   service_->RegisterPresentationClientMock(this);
 }
 
 MockPresentationClient::~MockPresentationClient() {
   DCHECK(!controller_);
-  if (service_)
-    service_->UnregisterPresentationClientMock(this);
+  DCHECK(service_);
+  service_->UnregisterPresentationClientMock(this);
 }
 
 void MockPresentationClient::SetScreenAvailability(bool available) {
   if (screen_availability_ == available)
     return;
   screen_availability_ = available;
 
   if (!controller_)
     return;
   if (!controller_->isAvailableChangeWatched())
     return;
   controller_->didChangeAvailability(screen_availability_);
 }
 
 void MockPresentationClient::Reset() {
   screen_availability_ = false;
-  service_ = nullptr;
 }
 
 void MockPresentationClient::setController(
     blink::WebPresentationController* controller) {
   DCHECK(controller_ != controller && (!controller || !controller_))
       << "Either one of the new or the old controller must be null, "
          "controller_ is " << controller_ << ", controller is " << controller;
 
   controller_ = controller;
 }
 
 void MockPresentationClient::updateAvailableChangeWatched(bool watched) {
   if (!watched)
     return;
   DCHECK(controller_);
   if (screen_availability_)
     controller_->didChangeAvailability(screen_availability_);
 }
 
 }  // namespace content