Improve documentation and tests for base::OnTaskRunnerDeleter.

content/public/browser/browser_thread.h

 // Copyright (c) 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.
 
 #ifndef CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 #define CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_
 
 #include <memory>
 #include <string>
 #include <utility>
@@ skipping 296 matching lines @@
   //
   // ...
   //  private:
   //   friend struct BrowserThread::DeleteOnThread<BrowserThread::IO>;
   //   friend class base::DeleteHelper<Foo>;
   //
   //   ~Foo();
   //
   // Sample usage with scoped_ptr:
   // std::unique_ptr<Foo, BrowserThread::DeleteOnIOThread> ptr;
+  //
+  // Note: when migrating BrowserThreads to TaskScheduler based
+  // SequencedTaskRunners these map to base::OnTaskRunnerDeleter.
   struct DeleteOnUIThread : public DeleteOnThread<UI> { };
   struct DeleteOnIOThread : public DeleteOnThread<IO> { };
   struct DeleteOnFileThread : public DeleteOnThread<FILE> { };
   struct DeleteOnDBThread : public DeleteOnThread<DB> { };
 
   // Returns an appropriate error message for when DCHECK_CURRENTLY_ON() fails.
   static std::string GetDCheckCurrentlyOnErrorMessage(ID expected);
 
  private:
   friend class BrowserThreadImpl;
 
   BrowserThread() {}
   DISALLOW_COPY_AND_ASSIGN(BrowserThread);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_BROWSER_THREAD_H_