certificate manager: Disable export option for TPM-backed certs.

chrome/browser/certificate_manager_model.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CERTIFICATE_MANAGER_MODEL_H_
 #define CHROME_BROWSER_CERTIFICATE_MANAGER_MODEL_H_
 
 #include <map>
 #include <string>
 
@@ skipping 83 matching lines @@
   // Returns true on success or false on failure.
   bool SetCertTrust(const net::X509Certificate* cert,
                     net::CertType type,
                     net::CertDatabase::TrustBits trust_bits);
 
   // Delete the cert.  Returns true on success.  |cert| is still valid when this
   // function returns.
   bool Delete(net::X509Certificate* cert);
 
   // IsHardwareBacked returns true if |cert| is hardware backed.
+  // This function is only implemented for Chrome OS and always returns false
+  // for other platforms.
   bool IsHardwareBacked(const net::X509Certificate* cert) const;
 
  private:
   // Callback used by Refresh() for when the cert slots have been unlocked.
   // This method does the actual refreshing.
   void RefreshSlotsUnlocked();
 
   net::CertDatabase cert_db_;
   net::CertificateList cert_list_;
 
   // The observer to notify when certificate list is refreshed.
   Observer* observer_;
 
   DISALLOW_COPY_AND_ASSIGN(CertificateManagerModel);
 };
 
 #endif  // CHROME_BROWSER_CERTIFICATE_MANAGER_MODEL_H_