Credential manager: Introduce platform interface.

public/platform/Platform.h

Index: public/platform/Platform.h
diff --git a/public/platform/Platform.h b/public/platform/Platform.h
index 4d134682a7b54a4d73459985428b32b6cb390f3c..fd7dd7fd632435fea77432a93a424f60e08d11c7 100644
--- a/public/platform/Platform.h
+++ b/public/platform/Platform.h
@@ -66,6 +66,7 @@ class WebClipboard;
 class WebCompositorSupport;
 class WebConvertableToTraceFormat;
 class WebCookieJar;
+class WebCredentialManager;
 class WebCrypto;
 class WebDatabaseObserver;
 class WebDiscardableMemory;
@@ -170,6 +171,9 @@ public:
     // Must return non-null.
     virtual WebBlobRegistry* blobRegistry() { return 0; }
 
+    // Credential Management -----------------------------------------------
+
+    virtual WebCredentialManager* credentialManager() { return 0; }
 
     // Database ------------------------------------------------------------