[Password Manager] Add UMA stats to track interaction with synced accounts.

components/password_manager/core/browser/password_store.h

Index: components/password_manager/core/browser/password_store.h
diff --git a/components/password_manager/core/browser/password_store.h b/components/password_manager/core/browser/password_store.h
index 6520f34da4b87a7e315adb20d1210ca65f10b0a4..b2549a3d89c53e231187dc8c4d2fa0daf3f18ad2 100644
--- a/components/password_manager/core/browser/password_store.h
+++ b/components/password_manager/core/browser/password_store.h
@@ -127,7 +127,9 @@ class PasswordStore : protected PasswordStoreSync,
       scoped_refptr<base::SingleThreadTaskRunner> db_thread_runner);
 
   // Reimplement this to add custom initialization. Always call this too.
-  virtual bool Init(const syncer::SyncableService::StartSyncFlare& flare);
+  // |sync_username| is specified to aid in metrics reporting.
+  virtual bool Init(const syncer::SyncableService::StartSyncFlare& flare,
+                    const std::string& sync_username);
 
   // Adds the given PasswordForm to the secure password store asynchronously.
   virtual void AddLogin(const autofill::PasswordForm& form);
@@ -169,7 +171,7 @@ class PasswordStore : protected PasswordStoreSync,
   virtual void GetBlacklistLogins(PasswordStoreConsumer* consumer);
 
   // Reports usage metrics for the database.
-  virtual void ReportMetrics();
+  virtual void ReportMetrics(const std::string& sync_username);
 
   // Adds an observer to be notified when the password store data changes.
   void AddObserver(Observer* observer);
@@ -203,7 +205,7 @@ class PasswordStore : protected PasswordStoreSync,
 
   // Methods below will be run in PasswordStore's own thread.
   // Synchronous implementation that reports usage metrics.
-  virtual void ReportMetricsImpl() = 0;
+  virtual void ReportMetricsImpl(const std::string& sync_username) = 0;
 
   // Bring PasswordStoreSync methods to the scope of PasswordStore. Otherwise,
   // base::Bind can't be used with them because it fails to cast PasswordStore