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

Add a separate boolean property to indicate that a client certificate
is hardware (TPM) backed. Certificate manager should disable the export
button for such certificates because there is no way to extract the
private key from the TPM.

BUG=126886
TEST=lumpy

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=138314

Review URL: https://chromiumcodereview.appspot.com/10407072

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@138595 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/browser/certificate_manager_model.h b/chrome/browser/certificate_manager_model.h
index 5196e36bd..cadeebd 100644
--- a/chrome/browser/certificate_manager_model.h
+++ b/chrome/browser/certificate_manager_model.h
@@ -101,6 +101,8 @@
   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: