Use GetSystemSalt from the cryptohome lib instead of reading salt off disk.

chrome/browser/chromeos/cros/cryptohome_library.cc

Index: chrome/browser/chromeos/cros/cryptohome_library.cc
diff --git a/chrome/browser/chromeos/cros/cryptohome_library.cc b/chrome/browser/chromeos/cros/cryptohome_library.cc
index 19d5f659368597167e66f3d1643351565ff205b2..5d0353089d2ce1a5f69bf2a94bdff718989714b9 100644
--- a/chrome/browser/chromeos/cros/cryptohome_library.cc
+++ b/chrome/browser/chromeos/cros/cryptohome_library.cc
@@ -23,4 +23,8 @@ bool CryptohomeLibraryImpl::IsMounted() {
   return chromeos::CryptohomeIsMounted();
 }
 
+CryptohomeBlob CryptohomeLibraryImpl::GetSystemSalt() {
+  return chromeos::CryptohomeGetSystemSalt();
+}
+
 }  // namespace chromeos