Use TaskScheduler instead of blocking pool in download_target_determiner.cc.

The following traits are used:

Priority: Inherited (default)
  The priority is inherited from the calling context (i.e. TaskTraits
  are initialized with the priority of the current task).

Shutdown behavior: SKIP_ON_SHUTDOWN (default)
  Tasks posted with this mode that have not started executing at
  shutdown will never run. However, any task that has already begun
  executing when shutdown is invoked will be allowed to continue and
  will block shutdown until completion.

  Note: Previously, the task was posted to the blocking pool with
  BLOCK_SHUTDOWN (default in SequencedWorkerPool).

May Block:
  Tasks posted with MayBlock() may block. This includes but is not
  limited to tasks that wait on synchronous file I/O operations:
  read or write a file from disk, interact with a pipe or a socket,
  rename or delete a file, enumerate files in a directory, etc. This
  trait isn't required for the mere use of locks.

BUG=667892

Review-Url: https://codereview.chromium.org/2625193002
Cr-Commit-Position: refs/heads/master@{#443232}

chrome/browser/download/download_target_determiner.cc

@@ -8,6 +8,7 @@
 #include "base/rand_util.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/stringprintf.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -567,9 +568,8 @@ DownloadTargetDeterminer::Result
     return CONTINUE;
   }
 
-  base::PostTaskAndReplyWithResult(
-      BrowserThread::GetBlockingPool(),
-      FROM_HERE,
+  base::PostTaskWithTraitsAndReplyWithResult(
+      FROM_HERE, base::TaskTraits().MayBlock(),
       base::Bind(&::IsAdobeReaderUpToDate),
       base::Bind(&DownloadTargetDeterminer::DetermineIfAdobeReaderUpToDateDone,
                  weak_ptr_factory_.GetWeakPtr()));