Start PopularSites fetch from separate function.

components/ntp_tiles/popular_sites.h

Index: components/ntp_tiles/popular_sites.h
diff --git a/components/ntp_tiles/popular_sites.h b/components/ntp_tiles/popular_sites.h
index 36237a391ab0970aef2ad11491d33ae38667e533..6be324843a0cd24639b5e47fac7ad2b4d484f39d 100644
--- a/components/ntp_tiles/popular_sites.h
+++ b/components/ntp_tiles/popular_sites.h
@@ -61,19 +61,23 @@ class PopularSites : public net::URLFetcherDelegate {
 
   using FinishedCallback = base::Callback<void(bool /* success */)>;
 
-  // When the suggestions have been fetched (from cache or URL) and parsed,
-  // invokes |callback|, on the same thread as the caller.
-  //
-  // Set |force_download| to enforce re-downloading the suggestions file, even
-  // if it already exists on disk.
   PopularSites(const scoped_refptr<base::SequencedWorkerPool>& blocking_pool,
                PrefService* prefs,
                const TemplateURLService* template_url_service,
                variations::VariationsService* variations_service,
                net::URLRequestContextGetter* download_context,
-               const base::FilePath& directory,
-               bool force_download,
-               const FinishedCallback& callback);
+               const base::FilePath& directory);
+
+  // Starts the process of retrieving popular sites. When they are available,
+  // invokes |callback| with the result, on the same thread as the caller. Never
+  // invokes |callback| before returning control to the caller, even if the
+  // result is immediately known.
+  //
+  // Set |force_download| to enforce re-downloading the suggestions file, even
+  // if it already exists on disk.
+  //
+  // Must be called at most once on a given PopularSites object.
+  void StartFetch(bool force_download, const FinishedCallback& callback);
 
   ~PopularSites() override;
 
@@ -104,19 +108,22 @@ class PopularSites : public net::URLFetcherDelegate {
   void ParseSiteList(std::unique_ptr<base::Value> json);
   void OnDownloadFailed();
 
+  // Parameters set from constructor.
+  scoped_refptr<base::TaskRunner> const blocking_runner_;
+  PrefService* const prefs_;
+  const TemplateURLService* const template_url_service_;
+  variations::VariationsService* const variations_;
+  net::URLRequestContextGetter* const download_context_;
+  base::FilePath const local_path_;
+
+  // Set by StartFetch() and called after fetch completes.
   FinishedCallback callback_;
+
   std::unique_ptr<net::URLFetcher> fetcher_;
   bool is_fallback_;
   std::vector<Site> sites_;
   GURL pending_url_;
 
-  base::FilePath local_path_;
-
-  PrefService* prefs_;
-  net::URLRequestContextGetter* download_context_;
-
-  scoped_refptr<base::TaskRunner> blocking_runner_;
-
   base::WeakPtrFactory<PopularSites> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(PopularSites);