Fix crash in GetPagesToServeURL

It is possible to reach this function in incognito mode, in which case
offline_page_model will be null. Added code to handle this condition instead
of crash.

Bug: 918119
Change-Id: Iddad53677e168c565a20dfc888d1ccb213a54fe7
Reviewed-on: https://chromium-review.googlesource.com/c/1393465Reviewed-by: Jian Li <jianli@chromium.org>
Commit-Queue: Dan H <harringtond@google.com>
Cr-Commit-Position: refs/heads/master@{#619662}

chrome/browser/offline_pages/offline_page_request_handler.cc

@@ -408,7 +408,11 @@ void GetPagesToServeURL(
       OfflinePageModel* offline_page_model =
           OfflinePageModelFactory::GetForBrowserContext(
               web_contents->GetBrowserContext());
-      DCHECK(offline_page_model);
+      if (!offline_page_model) {
+        FailedToFindOfflinePage(RequestResult::OFFLINE_PAGE_NOT_FOUND,
+                                network_state, job);
+        return;
+      }
       offline_page_model->GetPageByOfflineId(
           offline_id, base::Bind(&GetPageByOfflineIdDone, url, offline_header,
                                  network_state, web_contents_getter, job));