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

ios/web/public/web_thread.h

Index: ios/web/public/web_thread.h
diff --git a/ios/web/public/web_thread.h b/ios/web/public/web_thread.h
index 0ab786caa664456a5f132e6249cadadd10488b5a..be41b3080b58b14866953e8ab4464abf0c93c1da 100644
--- a/ios/web/public/web_thread.h
+++ b/ios/web/public/web_thread.h
@@ -139,6 +139,38 @@ class WebThread {
     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;