Add UMA histograms for tracking empty usernames.

components/password_manager/core/browser/password_manager.cc

Index: components/password_manager/core/browser/password_manager.cc
diff --git a/components/password_manager/core/browser/password_manager.cc b/components/password_manager/core/browser/password_manager.cc
index be4922c9ad42448492ddba246d1c9f1e81a3dd00..5574fb069886f08ffb926e20d148229a66b81d45 100644
--- a/components/password_manager/core/browser/password_manager.cc
+++ b/components/password_manager/core/browser/password_manager.cc
@@ -594,6 +594,10 @@ void PasswordManager::AskUserOrSavePassword() {
   RecordWhetherTargetDomainDiffers(main_frame_url_, client_->GetMainFrameURL());
 
   if (ShouldPromptUserToSavePassword()) {
+    bool empty_password =
+        provisional_save_manager_->pending_credentials().username_value.empty();
+    UMA_HISTOGRAM_BOOLEAN("PasswordManager.EmptyUsernames.OfferedToSave",
+                          empty_password);
     if (logger)
       logger->LogMessage(Logger::STRING_DECISION_ASK);
     if (client_->PromptUserToSavePassword(provisional_save_manager_.Pass())) {