Break cyclic dependency between CertDatabase and NSSCertDatabase.

net/cert/cert_database.h

Index: net/cert/cert_database.h
diff --git a/net/cert/cert_database.h b/net/cert/cert_database.h
index 865d685097d691e763c156c8cc714b995016f22f..1555b6fdce6d35abcd7615efa06a710b3db37781 100644
--- a/net/cert/cert_database.h
+++ b/net/cert/cert_database.h
@@ -16,8 +16,6 @@ template <class ObserverType> class ObserverListThreadSafe;
 
 namespace net {
 
-class NSSCertDatabase;
-
 // This class provides cross-platform functions to verify and add user
 // certificates, and to observe changes to the underlying certificate stores.
 
@@ -94,9 +92,11 @@ class NET_EXPORT CertDatabase {
 #endif
 
 #if defined(USE_NSS)
-  // Observe events from the |source| and forward them to observers of this
-  // CertDatabase.
-  void ObserveNSSCertDatabase(NSSCertDatabase* source);
+  // Returns an Observer which can be used to forward certificate events from
+  // another certificate database (currently NSSCertDatabase) to this database.
+  // Any On* method call on the observer triggers the same event on all
+  // observers of this database. The Observer is owned by this database.
+  Observer* GetObserver();
 #endif
 
  private: