Add a mojo interface for AccountId

components/signin/core/account_id/account_id.h

Index: components/signin/core/account_id/account_id.h
diff --git a/components/signin/core/account_id/account_id.h b/components/signin/core/account_id/account_id.h
index 4bc02054a86caede0d828110db4523693b789483..cc171b5c79e4265816c887cd41c524759a41a354 100644
--- a/components/signin/core/account_id/account_id.h
+++ b/components/signin/core/account_id/account_id.h
@@ -24,6 +24,7 @@ class AccountId {
  public:
   struct EmptyAccountId;
 
+  AccountId();

[msarda, 2017/01/09 10:28:50]

Why is this change needed?

[xiyuan, 2017/01/09 17:42:57]

This is needed for mojo deserialization. The code needs to create an AccoundId
instance using default ctor and pass it as output pointer to StructTraits::Read.

Think the original intention to make the default ctor private is to force using
one of the static FromXXXX method to construct an AccountId. This move does not
change that. The default ctor creates an invalid/emtpy AccountId (is_valid()
returns false for such instance). So any caller still has to fill it up with
proper data.

We probably can get rid of EmptyAccountId now after this move since the public
default ctor would provide the same capability. If you want, I can create a
follow up CL to get rid of that.

[msarda, 2017/01/10 15:04:22]

Thank you for the explanation. I am fine with us adding this constructor - I
think having the empty account id is also interesting in the log run.

May I ask you to add a comment about this constructor like:
// Creates an empty account id.
//
// Note: This constructor is public as it is required for mojo
// serialization. To create an AccountId object, prefer using the
// static FromXXXX methods or the EmptyAccountId method when
// creating an empty account id.

[xiyuan, 2017/01/12 00:06:54]

Done.

   AccountId(const AccountId& other);
 
   // If any of the comparable AccountIds has AccountType == UNKNOWN then it
@@ -90,7 +91,6 @@ class AccountId {
                           AccountId* out_account_id);
 
  private:
-  AccountId();
   AccountId(const std::string& id,
             const std::string& user_email,
             const AccountType& account_type);