Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/json_pref_store.cc

 // 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.
 
 #include "components/prefs/json_pref_store.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <utility>
@@ skipping 364 matching lines @@
             base::Bind(&JsonPrefStore::RunOrScheduleNextSuccessfulWriteCallback,
                        AsWeakPtr()),
             base::Callback<void(bool success)>(),
             base::SequencedTaskRunnerHandle::Get()));
   }
 }
 
 void JsonPrefStore::RegisterOnNextWriteSynchronousCallbacks(
     OnWriteCallbackPair callbacks) {
   DCHECK(CalledOnValidThread());
-  DCHECK(!has_pending_write_callbacks_);
 
   has_pending_write_callbacks_ = true;
 
+  // Note that this shouldn't clobber callbacks previously added by this method,
+  // as |writer_|'s WriteNow() will be called in sequence.

[gab, 2016/10/04 19:05:32]

I don't think this comment is required, it's never a problem to re-register
|writer_|'s write callbacks as they are moved with the write task (so if they're
overridden it's because they would never have been necessary anyways).

Looking at this some more though I think the confusion here comes from the two
bools |has_pending_successful_write_reply_| and |has_pending_write_callbacks_|.

Tracking |has_pending_write_callbacks_| is weird because it only becomes false
too late (i.e. on sequence, on the reply).

I think we can do with a single bool |has_pending_write_reply| which merely
indicates whether a PostWriteCallback task is scheduled currently.

The DCHECK at the top of RegisterOnNextSuccessfulWriteReply() can use
DCHECK(on_next_successful_write_reply_.is_null()) instead of its bool and the
rest of the logic can be tweaked slightly to make this work just as well with a
single bool.

[proberge, 2016/10/04 20:17:58]

Done.

   writer_.RegisterOnNextWriteCallbacks(
       callbacks.first,
       base::Bind(
           &PostWriteCallback,
           base::Bind(&JsonPrefStore::RunOrScheduleNextSuccessfulWriteCallback,
                      AsWeakPtr()),
           callbacks.second, base::SequencedTaskRunnerHandle::Get()));
 }
 
 void JsonPrefStore::ClearMutableValues() {
@@ skipping 203 matching lines @@
   DCHECK_EQ(31, num_buckets);
 
   // The histogram below is an expansion of the UMA_HISTOGRAM_CUSTOM_COUNTS
   // macro adapted to allow for a dynamically suffixed histogram name.
   // Note: The factory creates and owns the histogram.
   base::HistogramBase* histogram = base::Histogram::FactoryGet(
       histogram_name, min_value, max_value, num_buckets,
       base::HistogramBase::kUmaTargetedHistogramFlag);
   return histogram;
 }