Fix Presentation API mocks: do not drop eagerly drop clients.

BUG=412331

Review URL: https://codereview.chromium.org/938033002

Cr-Commit-Position: refs/heads/master@{#317057}

content/shell/renderer/test_runner/mock_presentation_client.cc

@@ -20,8 +20,8 @@ MockPresentationClient::MockPresentationClient(MockPresentationService* service)
 
 MockPresentationClient::~MockPresentationClient() {
   DCHECK(!controller_);
-  if (service_)
-    service_->UnregisterPresentationClientMock(this);
+  DCHECK(service_);
+  service_->UnregisterPresentationClientMock(this);
 }
 
 void MockPresentationClient::SetScreenAvailability(bool available) {
@@ -38,7 +38,6 @@ void MockPresentationClient::SetScreenAvailability(bool available) {
 
 void MockPresentationClient::Reset() {
   screen_availability_ = false;
-  service_ = nullptr;
 }
 
 void MockPresentationClient::setController(

content/shell/renderer/test_runner/mock_presentation_service.cc

@@ -29,7 +29,6 @@ void MockPresentationService::SetScreenAvailability(bool available) {
 
 void MockPresentationService::Reset() {
   FOR_EACH_OBSERVER(MockPresentationClient, presentation_clients_, Reset());
-  presentation_clients_.Clear();
   screen_availability_ = false;
 }