Back button for the WebView-based sign-in flow.

chrome/browser/resources/gaia_auth_host/authenticator.js

Index: chrome/browser/resources/gaia_auth_host/authenticator.js
diff --git a/chrome/browser/resources/gaia_auth_host/authenticator.js b/chrome/browser/resources/gaia_auth_host/authenticator.js
index ca068554e4ecc0dea6ab87a3bb0f23bc9987e221..e221f125659e120a71a099c25c1a0d7916a2601e 100644
--- a/chrome/browser/resources/gaia_auth_host/authenticator.js
+++ b/chrome/browser/resources/gaia_auth_host/authenticator.js
@@ -356,8 +356,10 @@ cr.define('cr.login', function() {
       this.dispatchEvent(new Event('dialogShown'));
     } else if (msg.method == 'dialogHidden') {
       this.dispatchEvent(new Event('dialogHidden'));
+    } else if (msg.method == 'backButton') {
+      this.dispatchEvent(new CustomEvent('backButton', {detail: msg.show}));
     } else {
-      console.warning('Unrecognized message from GAIA: ' + msg.method);
+      console.warn('Unrecognized message from GAIA: ' + msg.method);
     }
   };