Fix assert due to threading issues in update_client::ActionRunner.

The change to use task scheduler in update_client has incorrectly made
some of the update_client::ActionRunner code run on the UI thread,
therefore, the unpacker code is asserting that IO is not allowed.

See change https://chromium-review.googlesource.com/c/chromium/src/+/770050

Bug: 740992
Change-Id: I27a7ce650a1bf06ba76aac913d741c9adf5f6424
Reviewed-on: https://chromium-review.googlesource.com/792333Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Commit-Queue: Sorin Jianu <sorin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519786}

components/update_client/action_runner.cc

@@ -13,9 +13,7 @@
 #include "base/files/file_util.h"
 #include "base/location.h"
 #include "base/logging.h"
-#include "base/single_thread_task_runner.h"
-#include "base/threading/sequenced_task_runner_handle.h"
-#include "base/threading/thread_task_runner_handle.h"
+#include "base/task_scheduler/post_task.h"
 #include "components/update_client/component.h"
 #include "components/update_client/task_traits.h"
 #include "components/update_client/update_client.h"
@@ -37,8 +35,9 @@ void ActionRunner::Run(Callback run_complete) {
 
   run_complete_ = std::move(run_complete);
 
-  base::SequencedTaskRunnerHandle::Get()->PostTask(
-      FROM_HERE, base::BindOnce(&ActionRunner::Unpack, base::Unretained(this)));
+  base::CreateSequencedTaskRunnerWithTraits(kTaskTraits)
+      ->PostTask(FROM_HERE,
+                 base::BindOnce(&ActionRunner::Unpack, base::Unretained(this)));
 }
 
 void ActionRunner::Unpack() {