UMA to track the reason for re-auth (draft).

chrome/browser/resources/chromeos/login/screen_password_changed.js

Index: chrome/browser/resources/chromeos/login/screen_password_changed.js
diff --git a/chrome/browser/resources/chromeos/login/screen_password_changed.js b/chrome/browser/resources/chromeos/login/screen_password_changed.js
index 4f22fb833858dc3b10c35cc0e9f2f6436db1f75a..0bb4f3ab440f5677dfe20c1dd892efa512be2b8a 100644
--- a/chrome/browser/resources/chromeos/login/screen_password_changed.js
+++ b/chrome/browser/resources/chromeos/login/screen_password_changed.js
@@ -45,7 +45,8 @@ login.createScreen('PasswordChangedScreen', 'password-changed', function() {
 
       var gaiaPasswordChanged = $('gaia-password-changed');
       gaiaPasswordChanged.addEventListener('cancel', function(e) {
-          chrome.send('cancelPasswordChangedFlow');
+          chrome.send('cancelPasswordChangedFlow',
+              [$('gaia-password-changed').email]);
           gaiaPasswordChanged.reset();
         });
 
@@ -142,7 +143,7 @@ login.createScreen('PasswordChangedScreen', 'password-changed', function() {
      */
     cancel: function() {
       this.disabled = true;
-      chrome.send('cancelPasswordChangedFlow');
+      chrome.send('cancelPasswordChangedFlow', ['']);
     },
 
     /**