Fix GAPS cookie not being sent during reauthentication.

Manual repro steps:
1) start linux_chromeos's chrome with:
     --user-data-dir=/tmp/chrome --login-manager
2) login and close chrome
3) in "/tmp/chrome/Local State", replace OAuthTokenStatus's value from 4 to 3
4) start chrome again with same command line parameters and sign-in
5) the outgoing network request should have a Cookie header

Bug: 896862
Change-Id: I79b96ba071c3485578223a1a16ef6a48350c760f
Reviewed-on: https://chromium-review.googlesource.com/c/1289674
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600940}

chrome/browser/resources/gaia_auth_host/authenticator.js

@@ -251,6 +251,7 @@ cr.define('cr.login', function() {
         this.onHeadersReceived_.bind(this),
         {urls: ['<all_urls>'], types: ['main_frame', 'xmlhttprequest']},
         ['responseHeaders']);
+    this.onBeforeSetHeadersSet_ = false;
   };
 
   /**