Fix CFI bad cast errors involving OfflinePageModelImpl.

Make the class StubOfflinePageModel derive from
OfflinePageModelImpl. This fixes several test failures where code
in offline_page_model_factory.cc expects to be able to cast these
objects to type OfflinePageModelImpl.

Bug: 469376
Change-Id: I67b2d84cd2e2c549ceaa2bc8708bff478af30bd0
Reviewed-on: https://chromium-review.googlesource.com/812493
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522288}

components/offline_pages/core/stub_offline_page_model.h

@@ -9,16 +9,15 @@
 #include <string>
 #include <vector>
 
-#include "components/keyed_service/core/keyed_service.h"
 #include "components/offline_pages/core/client_policy_controller.h"
-#include "components/offline_pages/core/offline_page_model.h"
+#include "components/offline_pages/core/offline_page_model_impl.h"
 
 namespace offline_pages {
 
 // Stub implementation of OfflinePageModel interface for testing. Besides using
 // as a stub for tests, it may also be subclassed to mock specific methods
 // needed for a set of tests.
-class StubOfflinePageModel : public OfflinePageModel, public KeyedService {
+class StubOfflinePageModel : public OfflinePageModelImpl {
  public:
   StubOfflinePageModel();
   ~StubOfflinePageModel() override;