Revert of Add a default task runner argument to the constructor of...

Revert of Add a default task runner argument to the constructor of JsonPrefStore. (patchset #3 id:40001 of https://codereview.chromium.org/2876643003/ )

Reason for revert:
Broke compile step on ChromiumOS x86-generic Compile:

https://uberchromegw.corp.google.com/i/chromium.chromiumos/builders/ChromiumOS%20x86-generic%20Compile/builds/35463

Original issue's description:
> Add a default task runner argument to the constructor of JsonPrefStore.
>
> Callers that don't need to run tasks on the same sequence as the
> JsonPrefStore will be able to use the default argument.
>
> BUG=667892
>
> Review-Url: https://codereview.chromium.org/2876643003
> Cr-Commit-Position: refs/heads/master@{#471428}
> Committed: https://chromium.googlesource.com/chromium/src/+/22f197b4c0399140f9cf118136a8c96b8ad8af9a

TBR=gab@chromium.org,fdoray@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=667892

Review-Url: https://codereview.chromium.org/2884473002
Cr-Commit-Position: refs/heads/master@{#471450}

components/prefs/json_pref_store.cc

@@ -143,13 +143,13 @@ scoped_refptr<base::SequencedTaskRunner> JsonPrefStore::GetTaskRunnerForFile(
 
 JsonPrefStore::JsonPrefStore(
     const base::FilePath& pref_filename,
-    scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner,
+    const scoped_refptr<base::SequencedTaskRunner>& sequenced_task_runner,
     std::unique_ptr<PrefFilter> pref_filter)
     : path_(pref_filename),
-      sequenced_task_runner_(std::move(sequenced_task_runner)),
+      sequenced_task_runner_(sequenced_task_runner),
       prefs_(new base::DictionaryValue()),
       read_only_(false),
-      writer_(pref_filename, sequenced_task_runner_),
+      writer_(pref_filename, sequenced_task_runner),
       pref_filter_(std::move(pref_filter)),
       initialized_(false),
       filtering_in_progress_(false),

components/prefs/json_pref_store.h

@@ -19,7 +19,6 @@
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "base/observer_list.h"
-#include "base/task_scheduler/post_task.h"
 #include "base/threading/non_thread_safe.h"
 #include "components/prefs/base_prefs_export.h"
 #include "components/prefs/persistent_pref_store.h"
@@ -63,18 +62,13 @@ class COMPONENTS_PREFS_EXPORT JsonPrefStore
       const base::FilePath& pref_filename,
       base::SequencedWorkerPool* worker_pool);
 
-  // |pref_filename| is the path to the file to read prefs from. It is incorrect
-  // to create multiple JsonPrefStore with the same |pref_filename|.
-  // |sequenced_task_runner| is used for asynchronous reads and writes. It must
-  // have the base::TaskShutdownBehavior::BLOCK_SHUTDOWN and base::MayBlock()
-  // traits. Unless external tasks need to run on the same sequence as
-  // JsonPrefStore tasks, keep the default value.
-  JsonPrefStore(const base::FilePath& pref_filename,
-                scoped_refptr<base::SequencedTaskRunner> sequenced_task_runner =
-                    base::CreateSequencedTaskRunnerWithTraits(
-                        {base::MayBlock(),
-                         base::TaskShutdownBehavior::BLOCK_SHUTDOWN}),
-                std::unique_ptr<PrefFilter> pref_filter = nullptr);
+  // |sequenced_task_runner| must be a shutdown-blocking task runner, ideally
+  // created by the GetTaskRunnerForFile() method above.
+  // |pref_filename| is the path to the file to read prefs from.
+  JsonPrefStore(
+      const base::FilePath& pref_filename,
+      const scoped_refptr<base::SequencedTaskRunner>& sequenced_task_runner,
+      std::unique_ptr<PrefFilter> pref_filter);
 
   // PrefStore overrides:
   bool GetValue(const std::string& key,