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

chrome/browser/chromeos/cros/cryptohome_library.h

Index: chrome/browser/chromeos/cros/cryptohome_library.h
diff --git a/chrome/browser/chromeos/cros/cryptohome_library.h b/chrome/browser/chromeos/cros/cryptohome_library.h
index 16c5a90d9bb99a29031fbbd1e419d59cb8b54a22..9a3f028b9a3b078da96b525a25c6a185c6e8f5f1 100644
--- a/chrome/browser/chromeos/cros/cryptohome_library.h
+++ b/chrome/browser/chromeos/cros/cryptohome_library.h
@@ -31,6 +31,9 @@ class CryptohomeLibrary {
   // Asks cryptohomed if a drive is currently mounted.
   virtual bool IsMounted() = 0;
 
+  // Asks cryptohomed for the system salt.
+  virtual CryptohomeBlob GetSystemSalt() = 0;
+
 };
 
 // This class handles the interaction with the ChromeOS cryptohome library APIs.
@@ -48,6 +51,9 @@ class CryptohomeLibraryImpl : public CryptohomeLibrary {
   // Asks cryptohomed if a drive is currently mounted.
   virtual bool IsMounted();
 
+  // Asks cryptohomed for the system salt.
+  virtual CryptohomeBlob GetSystemSalt();
+
  private:
   DISALLOW_COPY_AND_ASSIGN(CryptohomeLibraryImpl);
 };