Integrate registry_hash_store_contents with the rest of tracked prefs.

components/user_prefs/tracked/hash_store_contents.h

Index: components/user_prefs/tracked/hash_store_contents.h
diff --git a/components/user_prefs/tracked/hash_store_contents.h b/components/user_prefs/tracked/hash_store_contents.h
index b55a6ebe83807cc82da064c9236f7f153d31e2a0..858664691abf7cf44fa08ca98d70e3998f717b38 100644
--- a/components/user_prefs/tracked/hash_store_contents.h
+++ b/components/user_prefs/tracked/hash_store_contents.h
@@ -14,6 +14,11 @@ class DictionaryValue;
 class Value;
 }  // namespace base
 
+enum class HashStoreContentsType : int32_t {

[gab, 2016/08/08 04:37:45]

Can get rid of this now?

[proberge, 2016/08/31 17:30:16]

Done.

+  DICTIONARY_HASH_STORE_CONTENTS = 1,
+  REGISTRY_HASH_STORE_CONTENTS = 2
+};
+
 // Provides access to the contents of a preference hash store. The store
 // contains the following data:
 // Contents: a client-defined dictionary that should map preference names to
@@ -24,6 +29,17 @@ class HashStoreContents {
  public:
   virtual ~HashStoreContents() {}
 
+  // Helper function to make a clone of this object.
+  // NOTE: this is intended to be used as a lightweight alternative to making
+  // this class RefCountedThreadSafe. Do not make copies of this object unless
+  // you can't avoid it.

[gab, 2016/08/08 04:37:45]

In order to make this API cleaner, I'd like us to add

// Returns true if this implementation of HashStoreContents can be copied via
MakeCopy().
virtual bool IsCopyable() const = 0;

and then I think we can make the documentation of MakeCopy():

// Returns a copy of this HashStoreContents. Must only be called on lightweight
implementations (which return true from IsCopyable()) and only in scenarios
where a copy cannot be avoided.

(note lack of use of "you"/"we" pronouns in comment too which is preferred)

[proberge, 2016/08/31 17:30:16]

Done.

+  virtual std::unique_ptr<HashStoreContents> MakeCopy() const = 0;
+
+  // Returns the string to append to UMA histograms for this store type.

[gab, 2016/08/08 04:37:45]

"Returns the suffix to be appended"

(the fact that it's a string is implicit IMO)

[proberge, 2016/08/31 17:30:16]

Done.

+  // The returned value must either be an empty string or one of the values in
+  // histograms.xml's TrackedPreferencesExternalValidators.
+  virtual std::string GetUMASuffix() const = 0;

[gab, 2016/08/08 04:37:45]

Return a base::StringPiece instead of an std::string to avoid forming a string
out of constants (i.e. avoid a copy).

[proberge, 2016/08/31 17:30:16]

Done.

+
   // Discards all data related to this hash store.
   virtual void Reset() = 0;