Revert of Remove unused blocking pool methods from web::WebThread.

ios/web/public/web_thread.h

 // Copyright 2014 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.
 
 #ifndef IOS_WEB_PUBLIC_WEB_THREAD_H_
 #define IOS_WEB_PUBLIC_WEB_THREAD_H_
 
 #include <string>
 #include <utility>
 
@@ skipping 121 matching lines @@
                                             std::move(task), std::move(reply));
   }
 
   template <class T>
   static bool DeleteSoon(ID identifier,
                          const tracked_objects::Location& from_here,
                          const T* object) {
     return GetTaskRunnerForThread(identifier)->DeleteSoon(from_here, object);
   }
 
+  // Simplified wrappers for posting to the blocking thread pool. Use this
+  // for doing things like blocking I/O.
+  //
+  // The first variant will run the task in the pool with no sequencing
+  // semantics, so may get run in parallel with other posted tasks. The second
+  // variant will all post a task with no sequencing semantics, and will post a
+  // reply task to the origin TaskRunner upon completion.  The third variant
+  // provides sequencing between tasks with the same sequence token name.
+  //
+  // These tasks are guaranteed to run before shutdown.
+  //
+  // If you need to provide different shutdown semantics (like you have
+  // something slow and noncritical that doesn't need to block shutdown),
+  // or you want to manually provide a sequence token (which saves a map
+  // lookup and is guaranteed unique without you having to come up with a
+  // unique string), you can access the sequenced worker pool directly via
+  // GetBlockingPool().
+  //
+  // If you need to PostTaskAndReplyWithResult, use
+  // base::PostTaskAndReplyWithResult() with GetBlockingPool() as the task
+  // runner.
+  static bool PostBlockingPoolTask(const tracked_objects::Location& from_here,
+                                   base::OnceClosure task);
+  static bool PostBlockingPoolTaskAndReply(
+      const tracked_objects::Location& from_here,
+      base::OnceClosure task,
+      base::OnceClosure reply);
+  static bool PostBlockingPoolSequencedTask(
+      const std::string& sequence_token_name,
+      const tracked_objects::Location& from_here,
+      base::OnceClosure task);
+
   // Returns the thread pool used for blocking file I/O. Use this object to
   // perform random blocking operations such as file writes.
   static base::SequencedWorkerPool* GetBlockingPool() WARN_UNUSED_RESULT;
 
   // Callable on any thread.  Returns whether the given well-known thread is
   // initialized.
   static bool IsThreadInitialized(ID identifier) WARN_UNUSED_RESULT;
 
   // Callable on any thread.  Returns whether execution is currently on the
   // given thread.  To DCHECK this, use the DCHECK_CURRENTLY_ON() macro above.
@@ skipping 76 matching lines @@
  private:
   friend class WebThreadImpl;
 
   WebThread() {}
   DISALLOW_COPY_AND_ASSIGN(WebThread);
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_PUBLIC_WEB_THREAD_H_