Fix TypeError in webRequestInternal.eventHandled

This CL is a followup to 1296539 and fixes a TypeError in
webRequestInternal.eventHandled() caused by this CL.
onHeadersReceived_ previously only returned headers when they contained
a SAML_HEADER, but the fix CL always returned all headers.

Bug: 900950, 887061
Change-Id: Ic39e2bd6a98c6e2e3411812f7cd90310c32e1d3b
Reviewed-on: https://chromium-review.googlesource.com/c/1314471Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606011}

chrome/browser/resources/gaia_auth_host/saml_handler.js

@@ -338,7 +338,7 @@ cr.define('cr.login', function() {
         }
       }
 
-      return details;
+      return {};
     },
 
     /**