Fix call to ReadingListDownloadService::CleanUpFiles.

If the application is shutting down, the task may run on a shutdowned
service.
Make the call static (so it does not need the service) and mark it
SKIP_ON_SHUTDOWN.
Also mark it USER_VISIBLE as it delays the offline feature of ReadingList.

TBR=olivierrobin@chromium.org

(cherry picked from commit a859436)

Bug: 762577
Change-Id: I05acf8e58b4dcdf9d21d5ee7e3f352d48f76f699
Reviewed-on: https://chromium-review.googlesource.com/654860
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#500592}
Reviewed-on: https://chromium-review.googlesource.com/668539
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/branch-heads/3202@{crosswalk-project#249}
Cr-Branched-From: fa6a5d8-refs/heads/master@{#499098}

Author: Olivier Robin

ios/chrome/browser/reading_list/reading_list_download_service.cc

@@ -40,6 +40,23 @@ const int kNumberOfFailsBeforeWifiOnly = 5;
 // Number of time the download must fail before we give up trying to download
 // it.
 const int kNumberOfFailsBeforeStop = 7;
+
+// Scans |root| directory and deletes all subdirectories not listed
+// in |directories_to_keep|.
+// Must be called on File thread.
+void CleanUpFiles(base::FilePath root,
+                  const std::set<std::string>& processed_directories) {
+  base::ThreadRestrictions::AssertIOAllowed();
+  base::FileEnumerator file_enumerator(root, false,
+                                       base::FileEnumerator::DIRECTORIES);
+  for (base::FilePath sub_directory = file_enumerator.Next();
+       !sub_directory.empty(); sub_directory = file_enumerator.Next()) {
+    std::string directory_name = sub_directory.BaseName().value();
+    if (!processed_directories.count(directory_name)) {
+      base::DeleteFile(sub_directory, true);
+    }
+  }
+}
 }  // namespace
 
 ReadingListDownloadService::ReadingListDownloadService(
@@ -146,27 +163,14 @@ void ReadingListDownloadService::SyncWithModel() {
     }
   }
   base::PostTaskWithTraitsAndReply(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::BACKGROUND},
-      base::Bind(&ReadingListDownloadService::CleanUpFiles,
-                 base::Unretained(this), processed_directories),
+      FROM_HERE,
+      {base::MayBlock(), base::TaskPriority::USER_VISIBLE,
+       base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
+      base::Bind(&::CleanUpFiles, OfflineRoot(), processed_directories),
       base::Bind(&ReadingListDownloadService::DownloadUnprocessedEntries,
                  base::Unretained(this), unprocessed_entries));
 }
 
-void ReadingListDownloadService::CleanUpFiles(
-    const std::set<std::string>& processed_directories) {
-  base::ThreadRestrictions::AssertIOAllowed();
-  base::FileEnumerator file_enumerator(OfflineRoot(), false,
-                                       base::FileEnumerator::DIRECTORIES);
-  for (base::FilePath sub_directory = file_enumerator.Next();
-       !sub_directory.empty(); sub_directory = file_enumerator.Next()) {
-    std::string directory_name = sub_directory.BaseName().value();
-    if (!processed_directories.count(directory_name)) {
-      base::DeleteFile(sub_directory, true);
-    }
-  }
-}
-
 void ReadingListDownloadService::DownloadUnprocessedEntries(
     const std::set<GURL>& unprocessed_entries) {
   for (const GURL& url : unprocessed_entries) {

ios/chrome/browser/reading_list/reading_list_download_service.h

@@ -70,10 +70,6 @@ class ReadingListDownloadService
   // not corresponding to a processed ReadingListEntry.
   // Schedules unprocessed entries for distillation.
   void SyncWithModel();
-  // Scans |OfflineRoot()| directory and deletes all subdirectories not listed
-  // in |directories_to_keep|.
-  // Must be called on File thread.
-  void CleanUpFiles(const std::set<std::string>& directories_to_keep);
   // Schedules all entries in |unprocessed_entries| for distillation.
   void DownloadUnprocessedEntries(const std::set<GURL>& unprocessed_entries);
   // Processes a new entry and schedules a download if needed.