Add the Sync.PassphraseType histogram.

This logs the passphrase type on sync startup if encryption is enabled.

BUG=421623

Review URL: https://codereview.chromium.org/779093002

Cr-Commit-Position: refs/heads/master@{#307094}

sync/internal_api/public/sync_encryption_handler.h

@@ -33,11 +33,14 @@ 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 if a value is added.
 enum PassphraseType {
   IMPLICIT_PASSPHRASE = 0,             // GAIA-based passphrase (deprecated).
   KEYSTORE_PASSPHRASE = 1,             // Keystore passphrase.
   FROZEN_IMPLICIT_PASSPHRASE = 2,      // Frozen GAIA passphrase.
   CUSTOM_PASSPHRASE = 3,               // User-provided passphrase.
+  PASSPHRASE_TYPE_SIZE,                // The size of this enum; keep last.
 };
 
 // Enum used to distinguish which bootstrap encryption token is being updated.

sync/internal_api/sync_encryption_handler_impl.cc

@@ -255,6 +255,10 @@ void SyncEncryptionHandlerImpl::Init() {
     WriteEncryptionStateToNigori(&trans);
   }
 
+  UMA_HISTOGRAM_ENUMERATION("Sync.PassphraseType",
+                            GetPassphraseType(),
+                            PASSPHRASE_TYPE_SIZE);
+
   bool has_pending_keys = UnlockVault(
       trans.GetWrappedTrans()).cryptographer.has_pending_keys();
   bool is_ready = UnlockVault(

tools/metrics/histograms/histograms.xml

@@ -36617,6 +36617,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Sync.PassphraseType" enum="SyncPassphraseType">
+  <owner>maxbogue@chromium.org</owner>
+  <summary>The active sync passphrase type at sync startup.</summary>
+</histogram>
+
 <histogram name="Sync.PasswordAssociationTime" units="milliseconds">
   <owner>zea@chromium.org</owner>
   <summary>
@@ -56365,6 +56370,13 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="3" label="Not migrated for an unknown reason"/>
 </enum>
 
+<enum name="SyncPassphraseType" type="int">
+  <int value="0" label="IMPLICIT_PASSPHRASE"/>
+  <int value="1" label="KEYSTORE_PASSPHRASE"/>
+  <int value="2" label="FROZEN_IMPLICIT_PASSPHRASE"/>
+  <int value="3" label="CUSTOM_PASSPHRASE"/>
+</enum>
+
 <enum name="SyncSimpleConflictResolutions" type="int">
   <summary>
     Sync simple conflict resolutions. The codes are listed in