[Offline Pages] Turn Offliner::Cancel into an async operation to resolve conflicting assumptions

chrome/browser/android/offline_pages/prerendering_offliner.h

Index: chrome/browser/android/offline_pages/prerendering_offliner.h
diff --git a/chrome/browser/android/offline_pages/prerendering_offliner.h b/chrome/browser/android/offline_pages/prerendering_offliner.h
index 9b1104cc57c095584df446246558d7c27dbe1107..935d3019f145d8a2c192c960d62bc5f9f8f43b7b 100644
--- a/chrome/browser/android/offline_pages/prerendering_offliner.h
+++ b/chrome/browser/android/offline_pages/prerendering_offliner.h
@@ -36,7 +36,7 @@ class PrerenderingOffliner : public Offliner {
   // Offliner implementation.
   bool LoadAndSave(const SavePageRequest& request,
                    const CompletionCallback& callback) override;
-  void Cancel() override;
+  void Cancel(const CancelCallback& callback) override;
   bool HandleTimeout(const SavePageRequest& request) override;
 
   // Allows a loader to be injected for testing. This may only be done once
@@ -76,6 +76,8 @@ class PrerenderingOffliner : public Offliner {
   // Listener function for changes to application background/foreground state.
   void OnApplicationStateChange(
       base::android::ApplicationState application_state);
+  void HandleApplicationStateChangeCancel(const SavePageRequest& request,
+                                          int64_t offline_id);
 
   // Not owned.
   content::BrowserContext* browser_context_;