Remove AdaptCallbackForRepeating() in ImportantFileWriter.

If posting the task fails then it's not an appropriate task runner. See
https://crrev.com/c/1572809/3/base/files/important_file_writer.cc#258
for more context.

Bug: 714018
Change-Id: I620bd978ec2a1d6b6219fa3d85ac6fbcd5563af2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580041
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654577}

base/files/important_file_writer.cc

@@ -255,19 +255,15 @@ void ImportantFileWriter::WriteNow(std::unique_ptr<std::string> data) {
     return;
   }
 
-  Closure task = AdaptCallbackForRepeating(
+  auto task =
       BindOnce(&WriteScopedStringToFileAtomically, path_, std::move(data),
                std::move(before_next_write_callback_),
-               std::move(after_next_write_callback_), histogram_suffix_));
+               std::move(after_next_write_callback_), histogram_suffix_);
 
-  if (!task_runner_->PostTask(FROM_HERE, MakeCriticalClosure(task))) {
-    // Posting the task to background message loop is not expected
-    // to fail, but if it does, avoid losing data and just hit the disk
-    // on the current thread.
-    NOTREACHED();
+  // If PostTask() returns false, it means that |task_runner_| isn't
+  // BLOCK_SHUTDOWN and thus isn't appropriate to write an important file.
+  CHECK(task_runner_->PostTask(FROM_HERE, std::move(task)));
 
-    task.Run();
-  }
   ClearPendingWrite();
 }