[TaskScheduler]: Use ScopedBlockingCall to mark blocking tasks.

This CL uses ScopedBlockingCall to mark blocking calls in /ios/chrome/browser/reading_list.

This CL was created by replacing calls to AssertBlockingAllowed()
with instantiations of ScopedBlockingCall(MAY_BLOCK).
I kindly ask the reviewer to make sure of the following:
  - ScopedBlockingCall is instantiated in a scope with minimal CPU usage.
    If this is not the case, ScopedBlockingCall should be instantiated
    closer to the blocking call. See scoped_blocking_call.h for more
    info. Please let me know when/where the blocking call happens if this needs
    to be changed.
  - Parameter |blocking_type| matches expectation (MAY_BLOCK/WILL_BLOCK). See
    BlockingType for more info. While I assumed MAY_BLOCK by default, that might
    not be the best fit if we know that this callsite is guaranteed to block.
  - The ScopedBlockingCall's scope covers the entirety of the blocking operation
    previously asserted against by the AssertBlockingAllowed().

This CL was uploaded by git cl split.

R=olivierrobin@chromium.org

Bug: 874080
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: If8096b4000f68e37625258c026af2db5f2b94a29
Reviewed-on: https://chromium-review.googlesource.com/1191787Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587972}

ios/chrome/browser/reading_list/reading_list_download_service.cc

@@ -14,7 +14,6 @@
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/string_util.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
 #include "components/reading_list/core/offline_url_utils.h"
 #include "components/reading_list/core/reading_list_entry.h"
 #include "components/reading_list/core/reading_list_model.h"
@@ -46,7 +45,6 @@ const int kNumberOfFailsBeforeStop = 7;
 // Must be called on File thread.
 void CleanUpFiles(base::FilePath root,
                   const std::set<std::string>& processed_directories) {
-  base::AssertBlockingAllowed();
   base::FileEnumerator file_enumerator(root, false,
                                        base::FileEnumerator::DIRECTORIES);
   for (base::FilePath sub_directory = file_enumerator.Next();

ios/chrome/browser/reading_list/url_downloader.cc

@@ -13,7 +13,6 @@
 #include "base/path_service.h"
 #include "base/stl_util.h"
 #include "base/task/post_task.h"
-#include "base/threading/thread_restrictions.h"
 #include "components/reading_list/core/offline_url_utils.h"
 #include "ios/chrome/browser/chrome_paths.h"
 #include "ios/chrome/browser/dom_distiller/distiller_viewer.h"
@@ -239,7 +238,6 @@ void URLDownloader::FetchPDFFile() {
 
 URLDownloader::SuccessState URLDownloader::SavePDFFile(
     const base::FilePath& temporary_path) {
-  base::AssertBlockingAllowed();
   if (CreateOfflineURLDirectory(original_url_)) {
     base::FilePath path = reading_list::OfflinePagePath(
         original_url_, reading_list::OFFLINE_TYPE_PDF);
@@ -296,7 +294,6 @@ URLDownloader::SuccessState URLDownloader::SaveDistilledHTML(
     const std::vector<dom_distiller::DistillerViewerInterface::ImageInfo>&
         images,
     const std::string& html) {
-  base::AssertBlockingAllowed();
   if (CreateOfflineURLDirectory(url)) {
     return SaveHTMLForURL(SaveAndReplaceImagesInHTML(url, html, images), url)
                ? DOWNLOAD_SUCCESS