Fix SHIFT+TAB order on GAIA login screens

BUG=488664
TEST=SHIFT+TAB repeatedly through login screen.

When focus is on the system tray, SHIFT+TAB should move focus onto the
[...] button, or whatever is the previous button on the shelf.
Previously, TAB and SHIFT+TAB would both move focus onto "Enter your
email" (or somewhere else in the GAIA elements).

Review URL: https://codereview.chromium.org/1173083003

Cr-Commit-Position: refs/heads/master@{#333803}

chrome/browser/resources/gaia_auth_host/authenticator.js

@@ -185,6 +185,7 @@ cr.define('cr.login', function() {
    * @param {Object} data Parameters for the authorization flow.
    */
   Authenticator.prototype.load = function(authMode, data) {
+    this.authMode = authMode;
     this.clearCredentials_();
     this.loaded_ = false;
     this.idpOrigin_ = data.gaiaUrl || IDP_ORIGIN;
@@ -322,7 +323,8 @@ cr.define('cr.login', function() {
    * @private
    */
   Authenticator.prototype.onFocus_ = function(e) {
-    this.webview_.focus();
+    if (this.authMode == AuthMode.DESKTOP)
+      this.webview_.focus();
   };
 
   /**