Make PassphraseType a "enum class" instead of "enum".

components/sync/core/sync_encryption_handler.h

Index: components/sync/core/sync_encryption_handler.h
diff --git a/components/sync/core/sync_encryption_handler.h b/components/sync/core/sync_encryption_handler.h
index 45aa304ef10f96a7697930ab8240c0c1855e9c81..3eb46046e9caefc4728d1e86c2b3512455df4be1 100644
--- a/components/sync/core/sync_encryption_handler.h
+++ b/components/sync/core/sync_encryption_handler.h
@@ -31,7 +31,7 @@ enum PassphraseRequiredReason {
 // the user should be prompted for a decryption passphrase.
 // Do not re-order or delete these entries; they are used in a UMA histogram.
 // Please edit SyncPassphraseType in histograms.xml if a value is added.
-enum PassphraseType {
+enum class PassphraseType {
   IMPLICIT_PASSPHRASE = 0,         // GAIA-based passphrase (deprecated).
   KEYSTORE_PASSPHRASE = 1,         // Keystore passphrase.
   FROZEN_IMPLICIT_PASSPHRASE = 2,  // Frozen GAIA passphrase.