Revert of Use TaskScheduler instead of SequencedWorkerPool in snapshot_cache.mm.

ios/chrome/browser/snapshots/snapshot_cache.mm

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/snapshots/snapshot_cache.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/critical_closure.h"
 #include "base/files/file_enumerator.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/mac/bind_objc_block.h"
 #include "base/mac/objc_release_properties.h"
 #include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/task_runner_util.h"
-#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_restrictions.h"
 #include "ios/chrome/browser/experimental_flags.h"
 #import "ios/chrome/browser/snapshots/lru_cache.h"
 #import "ios/chrome/browser/snapshots/snapshot_cache_internal.h"
 #include "ios/chrome/browser/ui/ui_util.h"
 #import "ios/chrome/browser/ui/uikit_ui_util.h"
 #include "ios/web/public/web_thread.h"
 
 @interface SnapshotCache ()
 // Returns the directory where the thumbnails are saved.
@@ skipping 533 matching lines @@
       FilePathForSessionID(sessionID, [SnapshotCache cacheDirectory]);
 
   // The color image may still be in memory.  Verify the sessionID matches.
   if (backgroundingColorImage_) {
     if (![backgroundingImageSessionId_ isEqualToString:sessionID]) {
       backgroundingColorImage_.reset();
       backgroundingImageSessionId_.reset();
     }
   }
 
-  base::PostTaskWithTraits(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_VISIBLE},
-      base::BindOnce(&ConvertAndSaveGreyImage, colorImagePath, greyImagePath,
-                     backgroundingColorImage_));
+  web::WebThread::PostBlockingPoolTask(
+      FROM_HERE, base::Bind(&ConvertAndSaveGreyImage, colorImagePath,
+                            greyImagePath, backgroundingColorImage_));
 }
 
 @end
 
 @implementation SnapshotCache (TestingAdditions)
 
 - (BOOL)hasImageInMemory:(NSString*)sessionID {
   if ([self usesLRUCache])
     return [lruCache_ objectForKey:sessionID] != nil;
   else
     return [imageDictionary_ objectForKey:sessionID] != nil;
 }
 
 - (BOOL)hasGreyImageInMemory:(NSString*)sessionID {
   return [greyImageDictionary_ objectForKey:sessionID] != nil;
 }
 
 - (NSUInteger)lruCacheMaxSize {
   return [lruCache_ maxCacheSize];
 }
 
 @end