Credential Manager: Add platform-exposed Credential objects.

Source/modules/credentialmanager/FederatedCredential.cpp

Index: Source/modules/credentialmanager/FederatedCredential.cpp
diff --git a/Source/modules/credentialmanager/FederatedCredential.cpp b/Source/modules/credentialmanager/FederatedCredential.cpp
index 61e1441da0bbf022579f8d35e823e01b9a4ad5b3..048073f4cd6ab5593b067666113f60f1e567627d 100644
--- a/Source/modules/credentialmanager/FederatedCredential.cpp
+++ b/Source/modules/credentialmanager/FederatedCredential.cpp
@@ -4,6 +4,7 @@
 
 #include "config.h"
 #include "modules/credentialmanager/FederatedCredential.h"
+#include "platform/credentialmanager/PlatformFederatedCredential.h"
 
 namespace blink {
 
@@ -13,11 +14,15 @@ FederatedCredential* FederatedCredential::create(const String& id, const String&
 }
 
 FederatedCredential::FederatedCredential(const String& id, const String& name, const String& avatarURL, const String& federation)
-    : Credential(id, name, avatarURL)
-    , m_federation(federation)
+    : Credential(PlatformFederatedCredential::create(id, name, avatarURL, federation))
 {
 }
 
+const String& FederatedCredential::federation() const
+{
+    return static_cast<PlatformFederatedCredential*>(m_private.get())->federation();
+}
+
 FederatedCredential::~FederatedCredential()
 {
 }