Add the Sync.PassphraseType histogram.

sync/internal_api/public/sync_encryption_handler.h

Index: sync/internal_api/public/sync_encryption_handler.h
diff --git a/sync/internal_api/public/sync_encryption_handler.h b/sync/internal_api/public/sync_encryption_handler.h
index cd1cfc04f02c498f5972abab79dd363584ce3cdf..9a9fd47d959950a8d6818e1abae398a8e668b9cb 100644
--- a/sync/internal_api/public/sync_encryption_handler.h
+++ b/sync/internal_api/public/sync_encryption_handler.h
@@ -33,6 +33,9 @@ enum PassphraseRequiredReason {
 
 // The different states for the encryption passphrase. These control if and how
 // 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 and increment the size
+// constant below if a value is added.
 enum PassphraseType {
   IMPLICIT_PASSPHRASE = 0,             // GAIA-based passphrase (deprecated).
   KEYSTORE_PASSPHRASE = 1,             // Keystore passphrase.
@@ -40,6 +43,8 @@ enum PassphraseType {
   CUSTOM_PASSPHRASE = 3,               // User-provided passphrase.
 };
 
+const int PASSPHRASE_TYPE_SIZE = 4;
+
 // Enum used to distinguish which bootstrap encryption token is being updated.
 enum BootstrapTokenType {
   PASSPHRASE_BOOTSTRAP_TOKEN,