Integrate registry_hash_store_contents with the rest of tracked prefs.

components/prefs/json_pref_store.cc

Index: components/prefs/json_pref_store.cc
diff --git a/components/prefs/json_pref_store.cc b/components/prefs/json_pref_store.cc
index e91a4063f7bf4307486017741afe8b666a27c25d..fe19f9573444ce9a8f6ee0a4f2b71b9faad8b1d7 100644
--- a/components/prefs/json_pref_store.cc
+++ b/components/prefs/json_pref_store.cc
@@ -382,10 +382,11 @@ void JsonPrefStore::RegisterOnNextSuccessfulWriteReply(
 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(