Explain the |kPasswordHashDataList| callback.

Currently, there is only one callback in the
hash_password_manager.h file and the comments
aren't clear enough. The callback is currently
only called when there is a change in the
|kPasswordHashDataList| and the change is not
associated to a first sign-in and the save
was successful.

Bug: 1016458
Change-Id: I422843144778a2e40eafd1f831576fae9a2d5fa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1872737
Auto-Submit: Bettina Dea <bdea@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708151}

components/password_manager/core/browser/hash_password_manager.h

@@ -57,7 +57,9 @@ class HashPasswordManager {
   void set_prefs(PrefService* prefs) { prefs_ = prefs; }
 
   // Adds a listener for when |kPasswordHashDataList| list might have changed.
-  // Should only be called on the UI thread.
+  // Should only be called on the UI thread. The callback is only called when
+  // the sign-in isn't the first change on the |kPasswordHashDataList| and
+  // saving the password hash actually succeeded.
   virtual std::unique_ptr<StateSubscription> RegisterStateCallback(
       const base::Callback<void(const std::string& username)>& callback);
 
@@ -68,7 +70,9 @@ class HashPasswordManager {
   PrefService* prefs_ = nullptr;
 
   // Callbacks when |kPasswordHashDataList| might have changed.
-  // Should only be accessed on the UI thread.
+  // Should only be accessed on the UI thread. The callback is only called when
+  // the sign-in isn't the first change on the |kPasswordHashDataList| and
+  // saving the password hash actually succeeded.
   base::CallbackList<void(const std::string& username)> state_callback_list_;
 
   DISALLOW_COPY_AND_ASSIGN(HashPasswordManager);